Possibly corrupted, although that usually shouldn't happen unless you were mid save when the machine crashed.
If I ever work in-house at a firm, the first thing I ever do is turn autosave on. I also progressive save projects on an approximately 3 hourly basis - even more often at the end of a project with a deadline. I also I run a back up to another disk at least once a day and upload the absolutely indispensible files to a web server once a day. None of this takes long and none of it is hard to do.
Computers crash, hard drives die, files get corrupted.
If it's any consolation, my first lesson to this effect was having a hard-drive die on me 2 days before the end of a three week project. I lost everything. Fortunately it was a deadline that was extendible and building the project the second time round could be done within a week or so. But it was a very stressful moment, the realisation that everything had gone.
That was my wake up call.
Be safe. Preferences -> Auto-save