Hi there,
Theres usually one of two fuses inside the microwave that can be changed, but you need to be VERY wary if opening it up as theres a big capacitor inside that stores a very high voltage.
This page is pretty detailed!...
http://repairfaq.ece.drexel.edu/sam/micfaq.htm
Good luck,
AJB.
AJB, April 2007