Howdy and welcome to iRV2.com.
Do you know if the compressor is running or not? If the hum is the compressor running and the fan does enot run, you may have a bad start capacitor on the fan motor or a bad fan motor.
With the A/C off and preferrably with the A/C breaker in the OFF position, remove the outside cover from the unit. Now, see if the condenser fan motor will move easily. If it rotates easily, have some one turn on the A/C breaker and stand by the A/C thermostat. Spin the fan by hand and tell them to turn on the A/C unit.
BE VERY CAREFUL around the open and exposed fan.
If the unit then runs normally, you probably have a bad fan capacitor, which can be found at some hardwear stores and A/C supply houses.
Be sure and kill the power to the u nit and short the terminals on the capacitor to drain the charge and not shock yourself.
Ken