"The file “NoteStore.sqlite” couldn’t be opened because it isn’t in the correct format The file might be corrupted, truncated, or in an unexpected format-notes went missing?

On creating/modifying a note here on 10.12.6; the most recent changes, and seemingly the entire content of the note, are in Home/Library/Group Containers/group.com.apple.notes/NoteStore.sqlite-wal

It's not likely, but I would look at that file just in case it has a date prior to your problem.

**edit**

Here, anyway; after deliberately corrupting NoteStore.sqlite : I got no error message, but Notes was empty...

However, a new file had been created, which did contain the previous information

Home/Library/Group Containers/group.com.apple.notes/NoteStore.sqlite.backup

If you have this, even with a newer date than the problem : make duplicates before doing anything else.

Do you see a Recently Deleted folder in Notes? Does this happen after a restart? Are you using Apple's Notes?

If it is still happening, try running the combo update. It installs all previous updates plus the new one, which can replace any files that may have become corrupted.

macOS Sierra 10.12.6 Combo

Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Select Library. Then try the above by PN2.

Open it & search for some specific text that you know should be there. Textedit works well enough here, albeit hard to read... but we're just searching.

If it looks promising, I'll have to get back to you on what to rename after testing, since originally, NoteStore.sqlite-wal contained my text, but now it's in NoteStore.sqlite.backup, while NoteStore.sqlite.backup-wal is empty.

That sounds a bit promising, but here at least, it's not as simple as changing the names to remove '.backup' then opening notes... that doesn't work.

If I find anything helpful, I'll post again.

Despite notestore.sqlite.backup containing text (but not images) of recent notes, and seemingly (from the name) being intended as a backup after some sort of failure : I can't find any way to make Notes use that information to recover the lost items.

Restoring good copies of NoteStore.sqlite & NoteStore.sqlite-wal works perfectly well... but unfortunately, that's of no use to you.

Do you have a backup?

Are they synch'd with iCloud? If so, are they available on iCloud.com?

Well, you're kind of in a bind, then.

Try moving all the NotesStore files to your Desktop, then start Notes and see if it recovers.