It must be either a heat pump or a wire has shorted out. Turn the power off and open the top, trace the wires from the fan and feel for bare spots. If you find one simply fold the wire and put a yellow wire nut on it.
If the short doesn't show up in that compartment, open the inspection panel on the side; sometimes a contactor will be welded in the closed position (from lightning or whatever).
Please post back and let us know how it comes out for you.