I had the same problem yesterday. Service technician says that it is one of three things that could be wrong (other online help sites seem to confirm this):
1) Thermostat has a circuit breaker that has tripped or the Thermostat is defective.
2) Thermister or Thermal Fuse had blown.
3) Thermostat/Gas Valve assembly is defective.
I am told that all of these parts can wear out more frequently if your dryer filter, vent, and vent piping are not kept VERY clean on a regular basis.
You can test the first problem on your own if you have a phillips screw driver and a common putty knife. Here's how:
First, UNPLUG THE ELECTRICITY!!
There are two clips that hold the top to the front. Slide a putty knife into the seam between top and front about two inches from the corners, push the clips backwards and pop the top up.
Next, remove two screws on top of the front. Then open the dryer door and remove the two screws on the front, just below the opening. (BE CAREFULL!! THE DOOR IS GLASS AND THE FRONT WILL FALL IF NOT SUPPORTED!!)
The reset switch in built onto the thermostat. The thermostat is located in the bottom right hand front corner of the dryer. It is behind another component, so it is not easily seen. You may want to use a small mirror to see it. Or just gently feel around for a small round pushbutton.
Once you have reset the switch, close the unit by reversing the steps above.
Test the dryer to see if it heats. If it fails again, then you have a defective part that will need replaced.
I hope this helps!
Roger
July 2008