Does the outside unit make any noise (like humming) at all? Does the compressor (behind the fan) start running when A/C is called, then shuts down or does it make no movement at all? The outside unit often has its own fuses in the disconnect box outside next to the unit, did you check those?
There can be several reasons for why the fan is not running. The motor's starting capacitor is one, the motors itself is another. If the unit is completely silent, it could also be the contactor that switches the motors on and off or the low voltage transformer that feeds it. Could also be either low or high pressure switch tripped which, in turn, can be a result of refrigerant leak.
If you suspect it can be the capacitor, you can take a stick or a long screwdriver and spin the fan. If it continues to run after that - it is the capacitor. If not, it's the motor itself or still either of the reasons above.
Post more information, including the manufacturer and model of the unit, it's always helpful.
There aren't many things a homeowner can fix in the A/C outside unit besides, perhaps the fan motor and its capacitor, so if the unit is absolutely silent and does not even try to turn on, I would suggest that you call an HVAC contractor for repair.