[DNSB]

I would recommend moving the calibre library outside of the Documents folder since Microsoft can and does play games with the contents of that folder such as a two way sync to OneDrive unless you have explicitly excluded it.

1. Close calibre
2. Move the Calibre Library directory outside of the Documents directory
3. Start calibre and create a new library. Use the Use a previously existing library at new location option and browse to your new library location. (i.e. "C:\Users\<username>\Calibre Library" Substitute your username for <username>.
4. Try the Restore database from library maintenance.
5. If this works and all your books show up happily, you're good to go.
6. Otherwise come back here for more help on recovering your library.