One of the most common reasons a microwave stops working is a tripped circuit breaker. This problem is easy to repair by checking the home circuit panel and resetting the breaker. Other common causes for the microwave to stop working include the door switch and the fan motor.
Electrical codes require circuit breakers to protect the wiring in the home from electrical overload. If the microwave or another appliance on the circuit has a short circuit, the breaker trips to interrupt the circuit. Plugging too many appliances into the same circuit has similar results. If none of the plugs on the circuit work, a bad breaker is the likely cause. If the appliance continues to cause the breakers to trip, it could require professional service.
The microwave has an interlocking door switch to protect users from microwave energy. With a good switch, a multimeter set to Rx1 reads infinity with the probes touching both ends of the switch leads if the door is open and zero when it is closed. If it shows any other reading, it requires replacement.
It is also possible to test the fan with the multimeter. With the fan unplugged from the unit and the meter set to Rx1, a reading of infinity indicates a bad fan motor. Check appliance parts stores for the appropriate replacement motor for the unit.