If, by any chance, you are using Onyx software version 1.7.5, and you feel able to use a command line solution, you can use the script in
https://www.mobileread.com/forums/sh...14&postcount=8 . I'm using it every few months, when i accidentally didn't leave the note app "properly" and it removed all notes again. From that i learned that when deleting notes, the app does not really delete them from the database, but just removes them from it's index file. They all reappear when using the script.
Best regards - Werner.