I had the same thing just happen to me. In fairness, I am running it through Wine in Linux because I couldn’t get the Linux package to work properly.
The problem occurred for me when I went from the Stream version to the Editor version. I made a backup before it got chewed up - it went from 7.1 MB to 3.4KB I’m not sure if that is the default size for an empty project?
I’m actually having a LOT of trouble with things, and I’m wondering if somewhere along the way part of the save file got malformed or something? I’m not too sure… but things like sprites no longer showing when they should be visible, or layer names not updating as I click between sprites… weird things like that. I’d be happy to show you a demo of the weird happenings I’ve got going on if you want to see them first-hand.