See my microwave oven safety file here
If it went dead almost immediately after pressing the Start pad, that's usually a shorted high-voltage capacitor. How to test a high-voltage capacitor
If it went dead several seconds after pressing the Start pad, that's usually a failing high-voltage transformer.
If it goes dead or blows the breaker (or GFI) when you plug it in or open or close the door, then there's a problem with a door switch or door switch mount. My door switch help file
If it's intermittent or random, it may be a bad connection, usually on the control board or a loose fuse holder, or even an intermittent fuse.
If the fuse is easy to twist with your fingertips while it's in the holder, it's too loose.
You should do a continuity test on the fuse while it's in the holder (with the microwave unplugged, of course) then turn the fuse by hand or take it out and put it back in, then test it again.
If you remove the fuse, then press the meter leads against the ends, it can allow internal contact to be made and make a bad fuse appear to be good.
FixYa does not directly notify us experts of responses, so if you have other questions, please contact me directly.
- William E. Miller, AS-EET
Microwave Oven Control Panel Repair Nationwide
"Recycling by repairing since 1982"