In the microwave oven, you will see a small bulb located behind the grill. It turns on when the microwave runs.
If during operation, the bulb does not turn on, it means it may have fused. Also, it may happen for bad wiring or faulty bulb socket.
Still, if you find everything fine, you should check the main control board to fix the issue.
Otherwise, you may contact a technician to fix this issue.