Diagnosis
The door latch assembly is misaligned or broken. The primary interlock switch is not being engaged, which prevents the unit from starting.
How to Fix It
Inspect the plastic latch hooks on the door and the corresponding strike plates on the cabinet frame. Look for cracks or deformation. A common failure is a broken plastic hook on the door itself. If broken, the door assembly must be replaced.
If That Didn't Work
The door hinge screws inside the top panel may have loosened, causing sag. Tighten these screws to realign the door. Do not overtighten.
Heads Up
Operating the microwave with a faulty door latch is a safety hazard. The unit will not operate if the primary interlock switch is open, which is a safety feature. Do not attempt to bypass it.