Replace the microwave if it a "stand alone" unit.be sure to vent it to the existing vent. Otherwise you will have a moisture buildup or worse. Aluminum vent pipes are available from Lowes Home Depot or similar store. The flat vents are about $20 and the other fittings,are a few dollars.
I replaced a keypad and a circuit board on a 6 year old microwave in 2008. As posted previously DO NOT open the unit unless you have knowledge of appliances. The parts were about $ 115 with shipping. Was a pain to track down the parts. The main reason I opted to repair was the microwave panel also controlled the exhaust fan and the exhaust fan would start automatically if the cook top passed a certain temp.