Why did my project revert to one week ago and I lost three days of work?

I don’t even know what to write here, because I doubt there can be any helping this and I can’t explain the scenario in which this occurred. But I worked on a file for days and now when I open it, it says in the MacOS finder info “last modified Nov 20th” which is a state of the file eight days ago, and the artwork in the file reflects that… I’ve lost three days worth of work that I’ve done since then. How does a file revert to a previous state? It doesn’t even make sense. I saved and closed and opened the file multiple times making progress on this file since the 20th. Usually I have a back up and save-as a new copy routine, but just this once I kept using the same file for days… and here’s what it got me.

I also teach with Harmony, and my students have mysterious lost drawings and progress regularly too. I guess I’m writing here in the hope that someone else has experienced this, so I know I’m not insane. Of course, any help would be great. But I’m not optimistic that I’m getting back any of that work.