Duo-Therm
Many of these units use one motor to turn both fans. That said, you could have a bad selector switch (off,fan,cool) that is not sending power to the fan, a loose wire, some foreign object jamming the fan blade, if the fan/cool selection is made at a wall thermostat, you may have a bad fan relay. Of course, it could also be a bad fan motor. The only way to know for sure is to check it out with a voltage/ohm meter. If you are not good at electrical things, find someone who is to do it for you (there should be a diagram inside the unit and/or in the manual). It would be well worth the few bucks you may have to spend to get someone honest.