in most cases these are cheaper to replace than repair, its could be a number of things, 1 the magnetron,2 the high voltage transfomer,or a number of other reasons, the problem is these are dangerous to work on and you would need to call for service, the cost would probably be more than just repalcing the microwave, please read this before going further .
Consumer Product Safety Commission
Electrocution Hazard with Do-It-Yourself Repairs of Microwave Ovens
CPSC Document #5061
The U. S. Consumer Product Safety Commission (CPSC) warns against do-it-yourself repairs of microwave ovens. Each year, on average, four people die from electrocution trying to repair their microwave ovens. Microwave ovens use high voltage which makes it particularly hazardous for consumers to remove the cover and touch electrical parts. The possibility for electrical shock from a microwave oven still exists even after the oven is disconnected from the power source. Should certain internal devices fail, even unplugged ovens can cause serious injury.
The Commission strongly recommends that consumers have professionals repair microwave ovens because electrical appliances
can be hazardous if consumers expose live electrical parts.
Removing the cover of your microwave oven and touching an electrical part can kill you.
Have a professional repair your microvave oven