Thanks for reaching out to us!
I don’t have good news for you unfortunately.
It is possible for teachers to delete ANY account, not just the ones that were created within their own classes. So it might have happened that one of your teachers accidentally deleted everybody… The students can rejoin, but unfortunately the contents they created will not be coming back. There is also a warning about this when you press delete.
We have already started to think about developing a feature that makes this impossible by having some kind of an administrator role introduced.
Sorry for not having better news.