All Unsynced iCloud Notes Disappeared from Macbook Pro

On my MacBook Pro (macOS Catalina 10.15.17), I had built up over a year's worth of notes on the "Notes" app. The notes were created in the "iCloud" folder but they did not transfer to the "On My Mac" folder even though the box for that was checked.

While I was trying to figure out how to transfer notes from my Mac over to my other devices through iCloud, I went to Apple ID > "Apps on this Mac using iCloud" and, on the advice of an online article on the matter, I un-checked and re-checked the "Notes" box.

When I did this, all of my notes in the "iCloud" folder that I had planned to sync to my other devices completely disappeared from my Mac.

None of them can be found online, nor on any of my other devices.

How do I get them back?