If the class you have been using with your students this semester suddenly disappeared, you can probably find it under "Past Classes" on your teacher dashboard.
If you have classes that are ongoing, but have an end date in the past, those are hidden under "Past Classes." You will need to look for the class there. Click the pencil icon on the "class card" to edit the class and change the end date to a date in the future when the class will actually end. That should move the class up to the "My Classes" section of the dashboard.
If your school is using a third-party like ClassLink or Clever, for rostering and managing classes, check with your admin at your school to confirm the class is set up properly and included in your school's data sync.
NOTE: This article applies to > Classroom Teacher Accounts