fan AND compressor not running or just the fan? either way, start with checking the capacitor first. check voltage to the contactor too, but you can simply check the breaker to see if it's tripped or not. let us know and we can direct you how to repair this if it's simply a capacitor.
if you open up your service panel of the condenser and look for a metal cyclinder or oval shaped can with wires coming out of the top. does it look blown up, rounded on top or is there possibly oily residue seeping from it? those are tell tale signs the cap is gone.