Had that once.  Ordered new heating element.  Went to install and the  air line outlet and and even the inlet screen needed cleaning.  Had to buy a vent cleaning kit. Cleaned and vacuumed everything I could get at.  I still haven't used the new heating element I ordered.

   There is an internal overheating breaker in there someplace on mine that kicks out and resets after it cools down.

  Also had it not heat one time this winter, but it must have just been the outside vent cover was froze shut as it has worked ever since.