What to do now?
If the URL looks right, first try refreshing the page, if it comes back here, then the URL is definitely not correct. Next: check the URL for errors (it's straightforward to get the 404 Not Found error because the URL was typed wrong, or the link that was clicked on has a spelling mistake.
Search for the page you were looking for...
Move up one directory level at a time in the URL until you find something. For example, if
/page/a/b/c gave you the 404 Not Found error, move up to
"/page/a/b/. If you get nothing here, move up to
/page/a/. This should lead you toward what you're looking for or at least confirm that it's no longer available.
If you move all the way up to the main page, try to run a search for the information you're looking for.
Perhaps we've made a mistake?
If all else has failed and you're quite sure that the resource you're looking for should exist at the URL you have, it would suggest that you've unearthed a bug in the site, so it would be most appreciated if you could quickly jot down a note to let us know what page you were trying to reach and specifically where you originally got the address for this link.