Chances are you have a bad high voltage parts, such as the high voltage rectifier diode, a bad magnetron, a loose connection at the magnetron, or less likely, a bad high voltage transformer.

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
http://www.gallawa.com/microtech/xformer.html

To order parts or find your nearest authorized servicer, please use the contact information in your owner's manual.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.