I created my entire project today (16-May). I saved locally and often, about every 5 minutes. I went back to reopen the file and it's saying it's invalid or corrupted. I've been working in SL360 all week and this is the only time I've had an issue.
I did the "create a copy and reopen" trick, and it didn't work.
Also, I don't seem to have access to the AppData on my PC because it's a work computer.