If your Classes and Assignments have disappeared, your Class may have been auto-archived because you likely have the class 'End Date' set for a date in the past. Once your Class reaches the 'End Date' it will auto archive. You can edit the 'End Date' to a date far enough in the future that will cover you for the whole class period and then un-archive the class. Once you do this, all of your assignments will return.
To do this:
1. Click 'Classes' and then 'Archived'
2. Click 'View' next to your class
3. Click the settings gear icon and then click 'Edit'
4. Edit the 'End Date'
5. Uncheck 'Archived' and click 'Save'