A number of different internal component problems could cause this heating failure in your microwave. It could be the magnetron, the high voltage transformer, the PC board, the diode, and the capacitor. This type of problem will need to be diagnosed and repaired by a service technician. Microwaves are inherently dangerous to service. This type of repair is not something that you want to try to do yourself. Internal capacitors in the microwave can retain a lethal electrical charge even when the microwave is completely unplugged. A microwave radiation leakage test must be performed on the unit using proper testing equipment following any internal component repair. These are the reason that I recommend calling a service technician to diagnose and repair this microwave failure.