Most of the time, once a trope is forgotten, it's dead and buried, fading from the memory of both audiences and creators; it is never to be seen again except in fossil form, or occasionally trotted out for an intentional Period Piece.
It is possible to resurrect a Forgotten Trope. A sufficiently successful work, Society Marching in a circle, or some other radical change in the arts or society at large may bring a long-dead trope back to relevancy. It usually happens without fanfare or warning -- thanks to whatever stimulus is involved, a trope that no one has thought of or used in years is suddenly found doing vigorous backstrokes in the meme pool. Other creators take notice and start turning out new works incorporating the revived trope, and once again it is part of the collective consciousness of the popular culture. It may even displace whatever subversion or replacement trope took its place, discrediting it and sending it into its own death spiral if they cannot exist side-by-side.
Just how successful a resurrection is may vary -- if the trope is of narrow enough scope there may not be enough works created over the long term to sustain it and it may well fade away again. But if the revived trope taps deeply enough into the new pop culture into which it's been reborn, it may find itself granted a new and long life. Only time will tell.
- Boarding School: This trope originally died in the 1960s as private schools became seen as elite and snobbish, and their students cast as enemies rather than heroes. However, with the culture-wide impact of J.K Rowling's Harry Potter novels it has been rather dramatically revived.
- High Times Future: For many years this was somewhere between a Discredited Trope and a Forgotten Trope, what with Drugs Are Bad having been draconically enforced through the 2000s. However, as marijuana legalization continues to gain traction within the United States (as of this writing numerous states have legalized it for medical purposes, and some have even given the stamp of approval to recreational use), it may be on the verge of Resurrection.
- Rags to Riches: After fading away entirely by the end of the 19th Century, this trope has been pretty much rescued thanks to the advent of the lottery (the good kind, not the Lottery of Doom). There are countless Real Life examples such as Oprah Winfrey and "Dot Com" success stories that offer a Real Life Deconstruction and object lesson of sorts. Often though, there is a sour grapes Aesop at the end of modern versions of these tales. The newly wealthy person realizes that money has corrupted them and they give it all up to return to a simple life.