Most likely the magnetron has quite working.
A magnetron is the piece that produces the microwave energy that heats the food. Most manufactures have a 5 yr warranty on the magnetron but labor is not included in the warranty.
I do not recommend you replace the part yourself as Microwaves carry LETHAL voltage even when unplugged.
I can not stress this enough, do not touch the inside unless you know exactly what you are doing.
If the microwave is an older model, its probably a good time to purchase a new one as they are quite inexpensive nowadays.
Chris
September 2005