Hey Mark, I could be wrong, but I believe that you have a 120vac breaker that feeds the A/C unit and any step down voltage would be handled by internal components.
1. Did you verify the 120vac breaker is not tripped?
2. Did you wait at least 5 min. after setting the thermostat to come on. You may have a built in 5 min timer to prevent short cycling of the compressor. ***This should not prevent Heater from coming on...but Heater is 12vdc power (check fuse).
3. Did you try turning the fan switch from Auto to On...does the fan run? If it does, then AC power is good to unit and problem could be one of the controls. (Bad capacitor)
Give us an update once you try some of these please.