Most refrigerator’s have a defrost timer this timer is what sends power to the compressor and both fan’s. Most newer models use a control board to start the fans and compressor, if your refrigerator has a board it will be inside the refrigerator, or on the back, could be underneath to.
Testing the voltage to the fan motor will verify what part has failed. The fan should receive 110v from the timer or board, if voltage is present the fan motor is bad.
Replacing a refrigerator fan motor is simple, remove the freezer back panel to access the fan, with the fan in hand swap over the bracket and re assembly, yes that simple.
If the fan does not have power present the board or broken wire will most likely be the culprit, refrigerators are simple to work on with the correct tools.