Notes stored “on my Mac” disappeared after checking box “Notes” of the iCloud account settings

I store my notes “on my Mac” because my iCloud has been full for 6 years.

This morning I checked the iCloud option for “notes”. My notes application was open in the background and I saw that all my notes (about 250) vanished. When I unchecked they didn’t re-appear.

The “on my Mac folder” is empty and there are no recently deleted files. It seems the NotesStore.sqlite was updated the moment I checked the iCloud option to sync notes.

All my notes from the past 6 years have disappeared from macbook. Some of my notes are very important. How can I recover them?