This might be bad news. Sounds like Unity had an issue opening the project? Are the files/assets still in the folder locations? Unity doing a re-import of assets isn't a bad thing, but it might mean you'll need to reconnect everything back up again (and maybe adjust any missing settings; with UFE might mean complete move data ).
This is a reason why you should do regular backups of your project. It's one of the main reasons I'm on Mac, so I can just let Time Machine handle all that stuff for me. I've had to revert some projects to a backup copy a few times now.
I also highly recommend some form of source control, either SVN or Git. SVN is better if you also want to backup non-code files, but Git is fine if you just want to back up your code/UFE assets (Global/Character/Move/AI files).
I also have faced the same problem like StriderSpinel's problem.
Our project is ready for completion , but at last i have faced the some unknowing problem at random time.
1) Some times intro/octro not worked proper and animations was stucked and particles position was changed and its look so much annoying for our project.
2) Some time automatic all music and FX was stoped , and after some time autometic it will works proper without any changes .
I was only restart game for 5 to 6 times then i was worked properlly.
Those both problems i was faced in Android device / iOS device and Unity Editor also.
I am trying to solve those problems but i can't get solution for those.
Other thing is that i work with iMAC, and i don't use any git application for backup.I am worked only in one of my iMAC only.So that i was backed up as local level after end of the day.
So please give us some guidance to resolve those problems.
Thanking you in advance.