I know it’s the last thing you want to hear when you lose all of your work but did you make a copy of the project? Any time I’m working in Thunkable, I make a copy of the project so that I have a backup of my work. And then I continually replace that backup with an updated copy as I make changes to my project.
Version history is another and better option once it’s out of beta. It’s really not useful right now with only one version available to us but I’m hopeful it will be improved/released soon.
For reference: Version history...?! and Versioning - Thunkable Docs