Diagnosis
The magnetron, high-voltage diode, or capacitor has failed. The turntable motor and control system work, but no microwave energy is produced.
How to Fix It
Perform a simple test. Place a cup of water in the microwave. Run it for 1 minute on High. If the water is not hot and the microwave hums normally, the high-voltage circuit is dead. Visually inspect the interior for any arcing marks or burned spots.
If That Didn't Work
The most common single component failure is the high-voltage diode. It can be tested for continuity with a multimeter (it should have continuity in one direction only). Replacing it requires discharging the high-voltage capacitor first.
Heads Up
The high-voltage capacitor holds a lethal charge even when unplugged. Only a qualified technician should diagnose and replace high-voltage components.