Louise may indeed be corrrect in her random-guess diagnosis - Wikipedia tells us that the components in the machine likely to cause the problem you have are...
a high voltage transformer, which passes energy to the magnetron
a cavity magnetron, which converts high-voltage electric energy to microwave radiation
a magnetron control circuit (usually with a microcontroller)
The door interlock switch may be defective or a fuse may have blown - so many choices...
Peccavi
July 2009