It sounds like the high voltage rectifier has shorted or the magnetron has failed.
There
should also be a "mini-manual" (tech sheet) hidden inside the unit behind the
control panel or hidden on the left side behind the grille, which is
very helpful when troubleshooting, testing, and locating components.
At
our Web site, we have a video available showing how to remove a typical
over the range control panel assembly in under 5 minutes.
You can usually find helpful exploded view diagrams
and order parts by entering your full model number
here.
Here are some links you or someone you know can use (in order given) for test help, but read the safety warnings first:
http://www.gallawa.com/microtech/diode.html (if it is NOT shorted, it's probably fine)
http://www.gallawa.com/microtech/mag_test.html (continuity can be good even though it is bad!)
http://www.gallawa.com/microtech/xformer.htmlWe're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.