Hello from JA My name is Woody. I will be glad to assist you today. I have been servicing ALL Major Brands of Appliances for 33 Years, and have over 100,000 in home repairs and over 50,000 phone and internet repairs under my belt.
If the microwave still operates you are fine .
If it is now dead ,no lights no nothing ,you have gotten the cavity thermostat hot enought to open up , and it will need to be replaced .
It is normally mounted above the cooking surface ,and will require the outer cabinet of the microwave to be removed .
If it mounts to the wall it will need to be removed off the wall to gain access to the cavity thermostat .