There are several reasons a fan may fail on a fan heater combination unit. The fan, rotor, and motor may be too dusty to work properly, so if you see a thick layer of dust, clean the fan and try again. If this doesn't help or the fan isn't dusty, then perform a close inspection of the individual parts. Look for loose bearings, sparks coming from the brushes, or an acrid odor that could indicate problems with the windings or that the motor needs lubrication. If you have a multimeter, then you can also check the motor for electrical resistance. If the motor is not receiving electricity, then there is likely a short circuit.