DIAGNOSIS A complete, sudden loss of power points to the thermal fuse (also called a thermal cut-off). It's a one-time safety device that blows if the unit overheats, often from a clogged vent or a failing magnetron cooling fan.
FIX You'll need to access and test the thermal fuse. Unplug the unit. Remove the outer casing (usually top and side panels) to get to the internal components. The thermal fuse is a small, white ceramic cylinder with two wires, typically located near the magnetron or the ventilation duct. Use a multimeter to test for continuity. If it shows 'open' (no continuity), it's blown and must be replaced with an identical part with the same temperature and current rating.
IF THAT DIDN'T WORK If the thermal fuse is intact, the next suspect is the main control board, which may have failed completely. Before condemning it, double-check all internal wiring connections for any that may have shaken loose.
HEADS UP A blown thermal fuse is a symptom. Before replacing it, you MUST find and fix the cause. Clean all vent paths (exhaust duct, cooling fan) thoroughly. If the cooling fan isn't running when the microwave is on, that motor needs to be replaced first, or the new fuse will just blow again.