If no class appears, select Join Class and enter the class invitation details from your teacher. See How do I join a class? for details.
If you joined with a different Google or Microsoft account, sign out and sign in with the account you used to join the class.
If teacher details or the student count are hidden, your teacher may have turned those details off in class options.
If a class disappeared, ask your teacher whether you are still enrolled in that class. If you intentionally left, you may need a new join link; see leaving a class.