All The Tropes:External Links Guide

Adding Links
If you add an external link, preview it to make sure it works.

If you add an external link to a site that has Not Safe For Work content (anything that would get an office worker or high-school student in trouble if the content or the site that hosts the content was to be viewed at work or school), then plainly mark the link as being Not Safe For Work.

Dealing with Link Rot
If you find an external link that doesn't work, add the  template immediately after the link to point it out. Don't delete the link; we'll usually want to attempt to find the page that was originally being linked.

If you know for a fact that the reason the link doesn't work is because the website has been taken down, replace it with a link that works! There's always the Wayback Machine, of course. If that doesn't help, Google it, even if you have no reason to expect results. You may be pleasantly surprised.