Deleted folder in Notes

You won’t be able to recover the folder itself but you’ll find the notes it contained in the “Recently Deleted” folder.

Recreate your original folder in the list of folders, select the deleted notes in the “Recently Deleted” folder (tap Edit and select the notes) and move them to the recreated folder (tap Move at the bottom and select the destination folder.)

Perhaps the folder wasn’t deleted but moved to another folder. Browse each folder to see if you can find it. To speed up the process, do a search on a note whose content you remember. The note location would appear in the search results.