It may have just stuck after not being moved for a while, but I would lay odds on burned switch contacts.

If you are mechanically handy, pull the control panel section on top apart, and check to see if the bimetallic has given up the ghost ( unlikely) or if the small limit switch actuated by the bimetallic popping at the set temp is the failed part.

The heaters are a pretty big load with a heavy draw when they first fire from being cold.
Oer time the contacts can arc on break and then the silver plating can wear away. Then the underneath copper or brass is susceptible to fusing on the break arc.