Check the compressor to ensure it is working. If the compressor is running but the fan is not, you might have a freon leak in the coolant portion of the compressor. This can't be replaced by people inexperienced in home repairs, so call in an expert. If the compressor is not running, the motor might have failed and should be replaced.
Check the condenser coil with a voltmeter, and then with a multimeter set to ohms. If the coil reads 120 volts, but the multimeter reads over 0 ohms, an open circuit is on the compressor and the pins in the relay might need to be replaced.
The defrost timer control can cause the failure of your evaporative fan. To tell whether the timer is faulty, confirm that the fan is not getting voltage with your voltmeter, and that the compressor is running.
Over time, the water vapor created during the cooling process can create rust or corrosion on the evaporative fan. If the fan is stopped and does not start up when you tap it, rust might have caused the motor to seize up and it should be replaced. Wear heavy work gloves to protect your hands when tapping the evaporative fan, just in case it kicks back on.