You've looked in "All" or "Screenshots" and the screenshots aren't there? I've never had that happen, so I can only assume that either the screenshots somehow got deleted or they weren't actually taken in the first place. Why don't you try going into a game, pressing c to take a screenshot, then checking to see if it's there? Then you'll know if the problem is that something happened to the old screenshots or if it's a bigger problem of not being able to take them and save them in the first place, if that makes sense.