To keep the navigation and rostering clean and simple when teaching, it's common to archive classes when the class ends (typically at the end of the year.) This removes the class from the class navigation for both the teacher and all the students in the class. Once removed, however, the experiences disappear for both the teacher and student, and can only be accessed via reporting. 

To see the data from any archived class, toggle the Show Archived Classes checkbox on. 

Any results from any class that has previously been archived will appear and will have a yellow "A" (for "Archived") badge next to it. If you roll over that class, you can see when it was created, and when it was archived. 

In the Experience Summary report, you can click on the hyperlinked experience name to view the actual experience, just as you can with non-archived classes.

This "Show Archived Classes" toggle appears on all relevant reporting for all relevant roles: teachers, campus-level administrators, and district-level administrators.