Games disappear from library

My students are beginners in Video Game Design class. 5 of them have worked diligently on their games only to have them disappear from their library. They have gone to their folders to find pieces of the files there, but nothing that is recoverable. They are logged in under their user name when this happens. In all cases they have worked on the game for several days with no problems, but they will come in and open UE4 and their game is gone with no trace. What are we doing wrong? They are signed in to their accounts and they are saving their games. I would sure appreciate your help because I can’t grade something that has disappeared, but I know it was there the day before.

Do a search for the .uproject file in their directory… If its there, double click on it and it should show back up in the launcher. If the uproject is missing, not sure because nothing that I know about the engine would delete files unless specifically told to either manually or by some sort of program.