Main Fuse did not open, only fuse for high voltage section was opened. Prior to the fuse blowing a louder than normal hum was heard from microwave for 1-2 seconds prior. Easy fix is the high voltage capacitor, so first checked the voltage across the capacitor terminals to ensure capacitor was discharged. Typically should be reasonably discharged within 30sec-1min assuming drain resistor is still in place. Removed the high voltage cap, and checked the capacitance and did not get a reading. Then checked the resistance across the terminals and found it was shorted with 1.5Ohms. Definitely bad capacitor. Diode checked out OK. Did not check anything on the magnetron and transformer since would require removing from wall.
Replaced Fuse and Capacitor, and microwave working again.