I use New Quizzes in my courses & have many questions in Item Banks. In early September, one of my students informed me that one of my quizzes wouldn't allow students to take the quiz. When I looked at the quiz, I found that one of my item banks (with over 50 questions) was missing. Upon further investigation, I found a total of 5 item banks missing from multiple courses. Each of the missing item banks had between 20 and 50 items in it, so I was not happy about the prospect of having to recreate everything from scratch.
I contacted Canvas Support about the missing item banks. They were able to recover the item banks after about 10 days.