DIAGNOSIS Complete power loss points to the thermal fuse or main control board failure. The thermal fuse is a primary safety component.
FIX Unplug the microwave. The 15-amp thermal fuse is located on the magnetron housing behind the control panel. Using a multimeter, test for continuity. A blown fuse will show no continuity. It must be replaced with an identical 15-amp, 250V thermal fuse.
IF THAT DIDN'T WORK If the thermal fuse is intact, the issue is the main control board or the power supply circuitry on it. This requires board replacement. Check for any visible burn marks on the board.
HEADS UP A blown thermal fuse indicates an overheating event. Before replacement, verify the cooling fan operates and the vents are not blocked. Simply replacing the fuse without addressing the cause will lead to a repeat failure.