Unplug the appliance, take the panel off the rear and you'll see the heater and 2 of the thermostats.
Check to see if there is a hole in the rear of one of them, if there is, use a pin to reset it.
If no, you'll need to test the thermostats to see if they are open circuit.
You'll need to replace both if one is open circuit.
You'll also need to find out why they tripped. The normal cause is poor airflow and a build up of fluff somewhere in the appliance.
Hope this helps