I thought I would post this just in case anyone else happens to have this issue as well.
One of my teachers that I gave access to set up classes emailed me saying that all of the classes were missing from the JSS. I figured it might have been a weird permission issue with their profile, but when I logged in myself I also did not see any.
I was searching around here and tried Googling as well but didn't see anyone else post specifically about this problem. I figured I would create a couple of dummy classes just to see if a delete all button might appear, but after I created the first class the rest of them reappeared.
Hopefully this is helpful to someone else, or maybe someone will have an explanation for the missing classes.
While I'm glad that adding a couple dummy classes brought the list back, if you haven't, I'd strongly suggest you get in touch with Support to get a case open.
The behavior you're describing is behavior we only typically see when there has been some corruption of data in the database; while adding some dummy classes may have brought the list back, the underlying problem likely still remains and may cause trouble in the future.
When you have the time to do so, please open up a case with Support so we can dig into it a bit deeper and try to find out what happened and check to see if there is any remaining corruption that may cause issues.