Just had this happen to me 
So awful, I’m curious as to how often this happens? I hate that I’m now mistrustful 
If you work on projects that are saved to an external drive, it happens a lot.
If you work on projects that are saved on an internal drive, it hardly ever happens (unless you run out of disk space).
Also, your .aup3 project file may be damaged. See: Audacity failed to read from a file in C: