The most common causes are a bad door switch or mount, a bad connection, or a bad high-voltage compnent - usually in that order.
If you or a friend decide to work on it, we have *critical* safety and disassembly info at our site, as well as other specific troubleshooting information and help.
In my professional opinion, people should not work on a microwave unless they are fully informed and aware of the deadly hazards and necessary precautions.
One can find such information and very specific in our safety and disassermbly text files at http://www.microwavedisplay.com
You can order parts for Sharp models from Tritronics at http://www.wehaveparts.com.
You can find wiring diagrams and parts lists and order parts for most other brands from the Pros here: http://tinyurl.com/gv383.
William Miller † MicrowaveDisplay.com, March 2007
