Assuming it's not a fuse, it's probably that little switch. I've had them go bad, or at least I think they've gone bad, only to have them start working again, although usually sporadically.
The switches are easy to replace. I'm going from memory here, since I'm too lazy to go out to my yard and look at one I have sitting there. This is all done from the inside. You just remove the little vent crank up handle by removing the screw holding it in place. You then remove the garnish, or interior trim ring, by removing the four screws holding it to the ceiling. The trim piece will come down with the switch attached, which you remove by removing the little ring which holds it on the trim piece. You should be able to access and disconnect the two power supply wires at this point.