Hi,Welcome to Just Answer, My name isXXXXX will be helping you with your appliance issue today.
The problem is the magnetron has failed in this, this is a job if you decide to repair it should be done by a technician only, microwaves and very dangerous to work on and this is on the high-voltage side of the microwave with a capacitor can store a lethal amount of voltage even at the machine is unplugged, below is the government warning on this,
Consumer Product Safety Commission
Electrocution Hazard with Do-It-Yourself Repairs of Microwave OvensCPSC 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 microwave oven
the magnetron itself runs under $50 at Sears , you're probably looking at about $80-$100 for service to replace it , hope this helps, thanks, XXXXX XXXXX and have a technician do this , Bryan