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Thanks for your swift reply. However, I was asking how to retrieve a copy of Notes.
Since you now type it with a capital N, do you mean the app itself, or the data? Assuming the latter:

I don't use the Notes app, but there's a database in Library/Group Containers/group.com.apple.notes called "NoteStore.sqlite". That may be the database of all notes.
 
If you are trying to reinstall the stock Notes app, boot your Mac via Command +R. When the Utilities screen appears, click on reinstall OS. Doing so should fix your problem while preserving your personal files.
 
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