air con compressor won't turn on

I'm working on a Dometic rooftop unit on a rental trailer. The renter left the air con running and the outside air temp took a nosedive into the 50's. The unit was making a loud humming noise, the blower was turning slowly, and the evaporator coils were frozen up. The unit would not turn off when I turned off the switch on the thermostat. I killed the breaker and let it thaw out for a while. Now it turns on and off, and the blower runs like it should, but the compressor does not turn on at all. I went up top, and found that the wire on top of the little black square WSX-7 Motor Starter was disconnected. I plugged it back in. Still the compressor does not start. The compressor is not getting any voltage on its 3 terminals.