That’s bad, sorry to hear that.
To try and figure out what happened:
- Most importantly, what sync solutions are you using? (Including if your vault is stored in a folder that your operating system automatically syncs, like OneDrive or iCloud.)
- What community plugins do you have installed? (If you aren’t willing or able to list them all, just take a look through them and think about which ones might be related to editing or syncing your notes.)
Regarding a log. I’m not sure. I think the only log you would see is if you were using Obsidian Sync. Otherwise if another tool changed your files, Obsidian would remain unaware of that. (As far as I know. If I’m wrong, hopefully someone else has an answer for you.)
Regarding mass-restoring. I think this is already a feature request. Right now I think the only mass-restore you can do is if you were keeping good incremental backups somewhere. (Something I highly recommend everyone should do.)