Have your electrical panel on the bottom open. While listening to the left hand power board, turn the left hand logic board mix potentiometer all the way clockwise. If not right away, within 10-15 seconds, you should hear a click and the mix refrigeration condenser fan should come on. If there's no click and the fan never comes on, check your hopper probe. If it is attached to other wires on the logic board, it should be separated for testing. Using an ohm meter, you should read between10K ohms (about 77 degrees) and 30Kohms (35 degrees) it should be in that range depending on whether you have anything in it or not. If its in this range and the potentiometer is turned all the way clock wise, then your powerboard is bad. If its a lot higher than 32K ohms, then most likey your probe is bad and the machine thinks its at temperature.
If the fan does come on then you will have to put refrigeration gauges on the mix refrigeration system. You should be reading 20PSI at the EPR valve and 1-10 PSI at the compressor. if you have those pressures, your compressor is working. if the pressures are equalized at anything above lets say 30PSI and theres no sign of change after a few minutes, then some component of the compressor has failed. it could be the relay, start capacitor, or the compressor itself. Of course if theres no pressure, you have a leak. Even with the proper pressures, you could be low on refrigerant. Check the whole system for leaks.