Disappearing Study Note Book

I was editing my notebooks and removing duplicates that spontaneously appear by themselves, and all of a sudden all of my 20+ notebooks were gone. I didn't click any thing to delete them, and to my knowledge I don't think it's even possible to delete all the notebooks at one time. How can I get my notebooks back?

Anything you delete should be found in your Unassigned Items folder or in your Trash. (Notebook items go to Unassigned, and Notes, Tags, Bookmarks go to Trash.) Have you checked there?