If it goes dead almost immediately after pressing the Start pad, that's usually a shorted high-voltage capacitor.
If it goes 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.
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.
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.
If you had posted your full model number from the tag (or at least the brand) I could have probably helped better.
You can find links to helpful exploded view diagrams and part ordering help
here.
I recommend that you bookmark that link to your favorites.
