Am replying to myself in case someone else encounters this phenomenon:
Notes are back - a glitch switched off Notes in Internet Accounts (my email address). Don't ask me why. Guided by an Apple support agent I switched it back on and all of my notes reappeared.
Prior to the above, I did the following procedure to restore Notes from a time machine backup (verified the procedure with an Apple support agent) - it turned out that the procedure was really unnecessary, but, if need be, here it is because it works:
macOS Monterey & Ventura: all notes missing from the Notes app
- Quit Notes app
- Disconnect physical internet connection
- Switch off WiFi
- Go to: ~/Library/Group Containers —> group.com.apple.notes
- Time Machine: restore suitable group.com.apple.notes
- Copy NoteStore files (ALL THREE SIMULTANEOUSLY) from group.com.apple.notes to desktop, then move them to trash (DON’T EMPTY TRASH)
- Reconnect internet connection
- Switch on WiFi
- Open Notes, verify content (should be empty), quit app
- Put back NoteStore files (ALL THREE SIMULTANEOUSLY), opt to replace existing ones
- Uncheck, then recheck iCloud —> Notes
- Open Notes (On My Mac), content should be restored
Hope it can be of help to someone.
Now I have only one more problem with Notes: although all is properly checked for the procedure, Notes won't backup to iCloud and I have no clue why. Though I am able to manually copy my notes to iCloud, I wonder: can I not have Notes on my desktop AS WELL AS a copy of it on iCloud that syncs as I edit my notes on the desktop? But this is stuff for another thread...