It hasn't happened to me but here is what I would do.

First, before doing anything else, make a backup of your save folder so you can always restore to the current state if "fixes" don't work.

Then, I would read through this post about restoring earlier saves. Hopefully it's not too late.

https://steamcommunity.com/app/892970/discussions/1/3409804177175439326/


I backup the entire save folder before every game session and have the save from the previous session as well. I never lose more than a few hours of progress.