Thank goodness for back-ups.
I haven't had anything like that, but because I never read the instructions, I thought half my book disappeared but it was because I had accidentally clicked the little arrow that precedes "chapter" in the outline menu collapsing the whole file.
Not that you would have done anything as computer illiterate as me.
For people who don't have back-ups, you can install recovery software that lets one recover corrupted and deleted files.
Another time we had a major failure we took the computer in for repairs and the fix-it geek was able to recreate the whole hard dive in a petitioned area of the drive and we had access to it again.
I have Time Machine running that makes hourly back-ups and I have 3 copies of most of my work on three separate flash drives, being that I'm paranoid.
If you find out what happened, I'd sure like to know since I'm using Scrivener.