Quite often this is due to a bad door switch.
Opening the door while cooking (without first hitting the STOP pad) is the primary cause of door switch failure.
I have a text file called "doorsw.txt" which helps in diagnosing and repairing door switch problems, and a file called "safety.txt" which is of vital importance.
In my professional opinion, no one should work on a microwave unless they are fully informed and aware of the deadly hazards and necessary precautions.
You can find this information and in our safety and disassembly text files at http://www.microwavedisplay.com
You can find safety and disassembly text files (as well as a text file which helps in diagnosing and repairing door switch problems) at http://www.microwavedisplay.com
You can find handy exploded views, wiring diagrams and parts lists and also order parts for most other brands from the Pros here: http://tinyurl.com/gv383.
William E Miller †MicrowaveDisplay.com
March 2007