I see that you got some help from a previous answer provided by Jacqueline S. I do not recommend that you open the microwave cabinet yourself unless you are a trained technician and know how to safely discharge a capacitor.
This type of internal component in a microwave should only be accessed by a technician. Internal high voltage components inside the microwave cavity can retain a lethal electrical charge even when the microwave is completely unplugged. A microwave radiation shielding test must be performed on the unit using proper testing equipment following any internal component access and/or repair. These are the reasons that I recommend having this type of failure serviced by a technician. Here is a link for the Sears Service website:
Sears Home Services