Lost Colony

A human colony on another planet experiences a disaster which destroys its tech-base, and for some reason the rest of humanity never checks up on that colony with which it suddenly lost all contact. Alternatively, a spaceship having nothing to do with colonization experiences an emergency which forces its crew to land on a planet in an uncharted star system, and for some reason they never get rescued.

Either way, not only do their descendants' politics, economics and culture regress to match their pre-Industrial Revolution technology level, they also forget that their ancestors ever came from another planet, making the story at first glance seem to be set in a pure fantasy world. Oftentimes whatever remains of the old technology will be mistaken for magic by the colonists' descendants. Sometimes there is genuine supernatural magic happening, or something that can pass for it, and all advanced technology has been lost. And sometimes the old technology and genuine magic are used side by side.

And of course, it's always popular to reveal that Earth was really a lost colony, though genetic evidence that humans and other Earth organisms are biologically related, all the way back to the first bacteria, has made this increasingly hard to sustain. Of course the theory of panspermia, that the first bacteria were dropped on Earth from space, still enables a scientifically plausible "Lost Colony" in a far more general sense.

These worlds are a popular venue for Planetary Romances.

Compare with After the End and Space Amish. Contrast Transplanted Humans.

WARNING: These examples necessarily contain spoilers.

Anime

 * Scrapped Princess.
 * Earl in Mai-Otome, while otherwise fitting this trope, at least remembers that the people and the Lost Technology came from Earth.
 * Trigun.
 * It's unclear whether Simoun is set on a Lost Colony populated by genetically altered humans, or whether they're humanoid aliens on their homeworld After the End.
 * Last Exile.

Music

 * US Heavy Metal band Iced Earth did a variant this with their Something Wicked arc. In it, humanity, a (very nearly) galaxy-spanning empire, came to Earth in order to conquer it. The natives, being at a clear disadvantage, decided to hide away a small fraction of their populace and initiate "The Clouding," completely wiping the memory of the entire human race, causing them to instantly revert back to the stone age. All of this in order to initiate a plan to wipe them all out. 10 000 years later. How a galaxy-wide empire was contained on a single planet, or, alternatively, why the rest of the galaxy-spanning empire didn't come looking for their friends, is never adequately explained.
 * Well, the Setians did sacrifice 10,000 of their people in order to power the Clouding effect, so its possible that it affected more than just one planet. As for the lost part, galaxy-spanning empires are always losing contact with colonies, hence this trope.

Literature
"Nearly 500 years before, a clerical error had erased from the computer's memory more than 1000 colonies, all in Sector F.U.B.A.R.3. For half a millennium, those colonies, young and struggling when last heard from, had had no contact with the rest of Humanity. The Galactic Patrol Interstellar Ship Hopeful, Captain Gregory Standforth commanding, was at once dispatched to reestablish contact with the Thousand Lost Colonies and return them to the bosom of Mankind."
 * Dragonriders of Pern is set on a Lost Colony, with the original colonists having used what remained of their technology to genetically engineer the telepathic, teleporting dragons which guard the world from the bicentennial rain of alien parasites.
 * It should be noted here that the rest of humanity DID send a ship to check it out, which found a small group who convinced the crew that everyone else had perished. The captain of the ship then quarantined Pern's system because of the Thread.
 * Also, Pern was intended from the start to be cut off from the rest of Humanity. the Pern colonists wanted to get away from the politics and war suffusing the rest of the Galaxy, and live a simple agrarian life with limited technology. Thread turned life on Pern into a struggle for survival, and caused them to lose the little bit of technology they intended to keep. The loss of knowledge of life before Pern was gradual; a book set in the Second Pass (roughly 150 years after the colonists arrived) mentions that computers still exist, but are becoming less practical as power and repairs get harder to come by. A book set in the Fourth Pass shows that the idea of Man coming to Pern from space is a theory rather than fact, and by the Seventh Pass (when the "main" series takes place, 2500 years after Landing), even said theories are completely forgotten.
 * There are few hints that Dragaera is a Lost Colony. If true, the colonists had landed at least 150,000 years before the start of the story, with all trace of the old technology long gone, making the use of the trope nothing more than a bit of flavoring.
 * Has happened at least twice, and very likely at least three times, to the planet on which the Saga of Recluce is set. Something about the planet acts like a magnet towards spaceships which have undergone malfunctioning hyperspace jumps.
 * In Larry Niven's Protector, Earth is actually a Lost Colony of a species called the Pak, who start life as a larval form (unintelligent hominids) and, in their thirties, are irresistibly drawn to eat a certain tuber which is host to a virus that transforms them into ageless hyper-intelligent killing machines (Protectors of their descendants). The colony failed because the tuber can't incubate the virus in soil deficient in thallium oxide (the tuber itself stayed, mutated, and is now known as the sweet potato), and the humans developed intelligence on their own. One Pak comes looking for the lost colony, kidnaps a human and turns him into a Protector, who is even smarter than the original variety (and acts very different). Eventually we learn that the Ringworld was built by Pak.
 * Niven's The Integral Trees series has a lost colony without a planet.
 * In Ursula K. Le Guin's Hainish sequence, Earth (and probably Gethen, too) were lost colonies of the oldest known inhabited world, Hain. Both may have started off as rather ethically suspect experiments, although the Hainish are very ethical these days. It's subtly implied that all the inhabited worlds may be Hainish in origin.
 * Erna in the Coldfire Trilogy is a colony from Earth, which they do know but are much too far from to take advantage of.
 * Also, the original colony has its own Lost Colony, although that was founded in a more low-tech manner (putting lots of people on ocean-going vessels). It turns out that
 * In The Kingdom Of Thorn And Bone by Greg Keyes, a part of the American colony of Virginia appears to be the ancestors of some of the characters.
 * Darkover by Marion Zimmer Bradley is born from the settlement by a lost ship from Earth.
 * Barrayar from the Vorkosigan Saga by Lois McMaster Bujold was a Lost Colony at a point of its history when the wormhole leading to it suddenly closed. However at the time when the series takes place, it's already been rediscovered through another route for three generations. It does still bear the barbaric mark of that time, however.
 * Interestingly, Artemis Fowl: The Lost Colony does not contain a traditional lost colony. In fact, the "colony" has forgotten magic in favour of technology, and possesses full knowledge of the outside world.
 * Christopher Stasheff's Warlock series has Gramarye, a colony founded by the Society for Creative Anachronism. In this case the loss of technology was deliberate.
 * Considering that the government of Earth became isolationist around the time Gramarye was settled, it is not surprising that they were not found for a long time. This same isolationist policy cut off all of earths colonies from the technological goods of the homeworld, leading many of them to lose all tech. They were not exactly lost colonies, since earth knew where they were in most cases, but it was practically the same thing. Although there were a few that earth had lost the paperwork for, come to think of it.
 * The setting of Mercedes Lackey's Bardic Voices books is implied to be a lost colony for at least one of the non-human species, the technologically advanced Deliambrens, and After the End for everyone else.
 * Part of the backstory of the Ea Cycle is that the planet Ea is a lost colony of the magitech-using "Star People". Ea later turns out to be Earth in the far future.
 * The Last Colony, though it was deliberately lost and recovered.
 * Vernor Vinge's Zones of Thought novels touch on Lost Colonies a great deal - in A Deepness In The Sky in particular, due to technological limitations and no Faster-Than-Light Travel, colonies tend to self-destruct on a regular basis.
 * L Sprague De Camp's The Great Fetish is set on the distant planet Kforri (K-40), a world in the Mesozoic stage of evolution colonized by humans generations before, their technology lost as the result of a mutiny before landing (or crashing). The established religions of the roughly Bronze Age-technology, Medieval European-culture nations that have developed all embrace the doctrine of Evolution, which states that mankind arose from lesser native forms, while the emergent scientists have brought forth the heretical concept of Descensionism, maintaining that the lack of creatures similar to humanity indicates their ancestors came from elsewhere. Hilarity ensues when the protagonist (a schoolteacher with Conanesque thews) runs afoul of the church by espousing the latter.
 * Hitchiker's Guide To The Galaxy. Humanity comes from a ship whose inhabitants thought they were to be a colony...their people were just annoyed with them and sent them off to an unknown fate. Things happened.
 * In David Brin's Uplift series, it is hotly debated whether humanity (uniquely among living peoples) developed intelligence independently or was created by some folk that abandoned the job incomplete.
 * In Virgin Planet by Poul Anderson, a shipload of women goes off course and, by necessity, founds a parthenogenetic society. Generations later, a male scout lands and is denounced as a monster, partly because he threatens the power of those who control the reproductive technology.
 * In Spartan Planet by A. Bertram Chandler, the lost colony is founded by a raging misogynist. Again, reproduction is artificial. As in ancient Sparta, babies with birth defects are left in the wilderness to die; here, lack of male parts is considered a defect.
 * Harry Harrison's novel Deathworld 2 is a Giving Radio to the Romans story set on a Lost Colony.
 * There are plenty of examples in The Stainless Steel Rat series. Makes sense, considering it occurs at a time when a collapsed Empire is being reassembled.
 * Keithland in The Cycle of Fire was founded when a ship was forced down on an uncharted world. All post-medieval technology was deliberately abandoned so their escaped alien prisoners wouldn't try to capture it.
 * Happens a lot in The History of the Galaxy novels. After the discovery of the Hypersphere anomaly, hundreds of colony ships were sent out to colonize other worlds. Many got lost due to the poorly-understood nature of the anomaly and lack of navigation equipment that works in another dimention. Throughout the series, many lost colonies are being rediscovered, many of which have regressed to barbarism and forgotten their origins. One short story details a demonstration of a new type of Space Fighter that involves dropping a cluster bomb onto a planet thought to be devoid of life. However, during the demonstration, a sensor on the planetary surface detects the presence of a human child, making the testers realize what a horrible mistake they've made. Luckily, a daring pilot manages to save the colony.
 * In Robert A. Heinlein's Time Enough For Love, brief mention is made of the discovery of a Lost Colony... founded by the surviving characters of Orphans Of The Sky.
 * Donald Westlake wrote a series of humorous short stories about the small starship Hopeful, crewed by a Ragtag Bunch of Misfits and given the task of contacting the colonies in an entire sector -- many of which had become Planets of Hats. From the prologue of the very first story:
 * Donald Westlake wrote a series of humorous short stories about the small starship Hopeful, crewed by a Ragtag Bunch of Misfits and given the task of contacting the colonies in an entire sector -- many of which had become Planets of Hats. From the prologue of the very first story:


 * Donald Kingsbury's Courtship Rite takes place on a lost colony so hostile to human life that over the centuries, times of famine have made cannibalism socially acceptable to varying degrees.
 * The planet He, in James Blish's Cities in Flight.
 * Margolia from Alex Benedict. The founder said he was going to travel so far away that not even God would be able to find them; and since no one heard from the colony over the next nine thousand years, he apparently succeeded.

Live Action TV

 * Most planets of the week in Stargate SG-1 and Stargate Atlantis are like this, populated by humans seeded by either the Ancients or their successors as dominant galactic powers, the Goa'uld and Wraith.
 * Atlantis itself was almost a lost colony during the first season as they were completely cut off from earth.
 * In Stargate Universe some members of the expedition attempted to start one of these.
 * And "now" an alternate timeline version of the Destiny crew founded an entire civilization two thousand years ago, which advanced to the point where they began using the stargates and slowships to colonize other planets. Unfortunately the colonies were then cut off when a supervolcano destroyed their homeworld and drones attacked their stargates, becoming lost colonies of a lost colony.
 * Played with in Babylon 5, where the rather human-looking Centauri originally claimed that Earth was a lost colony. When called on it, they excused it as a clerical error. "We thought your world was Beta 9, actually it was beta 12."
 * The truth was discovered when Humans got their hands on medical data regarding the Centauri and discovered that despite outward appearances, Centauri physiology is very different from Human physiology.
 * Used in spades in Battlestar Galactica. The Twelve Colonies are lost colonies of Kobol, whose civilization was destroyed by  The Colonists spend much of the series looking for Earth, Kobol's other lost colony, before discovering that  Ultimately,
 * A couple Star Trek episodes featured the re-discovery of a Lost Colony or two.

Tabletop Games

 * In the Humongous Mecha roleplaying game Mekton, the default campaign setting is a planet called Algol, which turns out to be a Lost Colony of the interstellar Space Opera society presented in the worldbook Mekton Empire.
 * Warhammer 40000 has these in abundance, sometimes simply because the massive bureaucracy that governs the Imperium loses records of worlds due to filing errors. In other cases, lost colonies that wound up on particularly challenging worlds are rediscovered and left in their primitive state because their inhabitants make such good soldiers, with the Imperium only occasionally popping in to recruit a company or two for the Imperial Guard. Others, such as Fenris, serve as Space Marine homeworlds, the recruits only learning of the existence of the wider galaxy after joining the Chapter.
 * Moreover, it used to be implied rather heavily that the Warhammer Fantasy Battle setting was an example of a world that had been cut off from Imperium contact, with hints that Sigmar and Bel'hakor were the two missing Primarchs. GW have subsequently moved away from this attitude however, treating the two game settings as being completely independent.
 * Implied? It was all but stated outright in the Liber Chaotica background books. At one point the narrator (a scholar from the Empire) has an out of body experience and briefly meets Magnus.
 * That doesn't necessarily mean the settings exist in the same universe. The Warp exists outside the normal universe, and may be shared by the two settings even if they are not in the same universe.
 * Deadlands: Lost Colony is, rather unsurprisingly, based on this trope - but with the notable difference that the colony has only been "lost" for a decade or so, because Earth had a slight apocalypse.
 * The source of the "barbarians" in Fading Suns.
 * In Eclipse Phase the Pandora Gates occasionally malfunction and cut off a planet for years. In one case the colonists formed a Hive Mind in the five years it took to re-establish contact.
 * This was used in the Star Fleet Universe as a way to insert a small bastion of humans in a section of space outside the main Federation region by causing a colony planet (with sun and all) to be transported about 30 degrees around the edge of the galaxy. Later joined by a Klingon penal planet (and the task force trying to restore order there).
 * The Cynidecians in the Dungeons and Dragons module The Lost City.

Video Games

 * All the Argon sectors in the X-Universe prior to X3: Terran Conflict are Earth's lost colonies. Earth and the Solar System are reunited to the X-Universe in Terran Conflict.
 * The Free State of Aldrin is also a lost Terran colony.
 * Karrak, the starting point for the player's fleet in Homeworld. The construction of the mothership started when archaeologists discovered the remains of the colony's landing site, and with it a hyperdrive module. By this point, the colony had spent a few thousand years re-developing sub-light space travel and a modern society, but they had lost all their history prior to the landing. The distant genesis theory had been heretical, but the remains of the ship proved it. The rather important detail that they missed was that they were the exiled losers of a galactic war, and using a hyperdrive will mark them for death by the all-encompassing Taiidan empire.
 * Homeworld is a subversion. The Taiidan knew all about the colony and, even after four thousand years, still monitered it. They just didn't announce their presence.
 * Xenogears: A select few in the world are aware of this; others, not so much. It was an accident to begin with, set ten thousand years prior to the start of the actual story, where mankind's creation crash-landed on the planet and respawned humanity through a fabricated "mother" entity. Furthermore, Xenosaga, the game's "spiritual" prequel, implies that the planet depicted at the end of the third game is the actual planet that Xenogears takes place on--though if they were to make a formal attempt to validate this (through sequels or otherwise), it would involve a fairly heavy Retcon.
 * Other interpretations state that  Either way, you have a ultra civilization dealing with "just still getting there."
 * The entire Koprulu Sector in Starcraft is a lost colony. Originally, Earth was dabbling with the concept of interstellar colonization, and already had success within the Solar System. One enterprising scientist built a fleet of colony ships, rounded up thousands of "colonists" (in reality convicted criminals because they were easy to obtain), and promptly shot them toward the nearest habitable planet. However, a freak accident caused the fleet to lose the coordinates of both their destination and Earth. They eventually were forced to stop in the Koprulu Sector. Eventually, the various human factions developed into small spacefaring empires. In the expansion, the United Earth Directorate finally discovers the lost colony and attempts to subjugate it. It doesn't end well.
 * The ARK in Sonic Adventure 2. A level set on the ARK is even called Lost Colony.
 * In Master of Orion 2, lucky exploration can net you with these. Finding a Splinter Colony effectively means instant colonization of a random, agriculture-supporting planet in the solar system.
 * Wild Arms 3
 * Some suspect that Seras is a Lost Colony and that the humans are alien invaders.
 * In Ys VI, Port Rimorge was founded by survivors of the Great Vortex who washed up on the islands over the years.
 * Large swathes of the Galaxy is, by in-game statements of sapients that would know, this in (old verse) Might and Magic, although we only get to see three of them (Enroth, Axeoth and the world of the novels) -- the four settings of the first five games were not lost, but deliberately retarded (they were part of Ancient experiments). Unusually, at least the inhabitants of Enroth do maintain knowledge that they came from another world (in fact, their dating system starts from the year off-world contact was lost), it just never comes up in the Heroes games.

Web Original

 * In Orion's Arm the exodus from Old Earth following the nanodisaster resulted in many Lost Colonies, though most were re-contacted when the First Federation was formed or sometime later in the next 10,000 years.
 * The Metasoft Version Tree maintains "baseline preserves" that are essentially intentional lost colonies. The original colonists were baseline humans who wanted to leave Sephirotic civilization and paid Metasoft for worlds where they could preserve their culture undisturbed. Though at least one preserve world has rediscovered space travel and rejoined interstellar civilization.

Western Animation

 * In The Real Adventures of Jonny Quest, an interesting variation appeared in the season one episode "Ice Will Burn". The story deals with a people descended from 17th/18th century Siberian Russians, whose ancestors had to fled their town and got trapped by accident in an unescapable deep gorge/icy cavern. They managed to survive and thrive thanks to the heat produced by a small local volcano (Truth in Television if it's supposed to be set somewhere in the Kamchatka peninsula).

Real Life

 * The adventurer-archeologist Aurel Stein found a Chinese outpost dating to the end of the Han dynasty that had been cut off by the collapse of the dynasty (as detailed in Romance of the Three Kingdoms). Ancient Bureaucratic red-tape indicated that the outpost continued to function after it had been cut off, still keeping up Imperial forms as if the dynasty still held the throne.
 * In 1843, German settlers in Venezuela founded Tovar. A few years later the jungle reclaimed the only way to them and everyone assumed they were all dead. Well, until they where rediscovered 1953. They still spoke their German dialect and built their houses like 19th century Germans.
 * DNA analysis has recently suggested that not one but four or five African and Indian groups may be (as they claim) Lost Tribes of Israel. Besides the famous Ethiopian and Ugandan Jews who got airlifted by Israel in the 70s, a full-blown tribe in Tanzania recently got themselves tested to prove their claim that the tribe itself were a lost tribe of Hebrews, since the Roman era. (Being cut off, they didn't know any other Jews existed, so they adopted local language and customs.)