Everything works, but it has stopped heating.
HI. Your symptoms are a strong indication of Magnetron failure. This is the principal heating device, that is used to produce microwaves that , essentially, excite the water molecules in the food, thus, producing the heating effect. When this device fails, generally, the unit will run, but not heat. This device is very fragile, and can injure you if you are not qualified to handle this type of component. There is a High Voltage Capacitor, that is used to help provide the necessary current for optimal Magnetron operation. This capacitor must be discharged correctly. If not, you could risk serious injury, during the repair, if it is accidental touched.
The Magnetron is linked to a cut-off fuse. If the magnetron becomes unstable, this fuse will shut the device down. It is standard procedure to have this fuse test for continuity. If the fuse has blown, this will be the culprit. Replace the fuse. Now, if the fuse continues to show continuity, this will confirm that the Magnetron has failed. I would advise to inspect the Capacitor, and HV diode for visible burn marks, or swollen top(Cap).
Access can be ac hived to these parts by removing the outside cabinet screws. Once the screws are removed, the cover can be removed, as well. Once the cover is removed, the internal components will be exposed for testing, and visual inspections.
WARNING: IF YOU ARE NOT A QUALIFIED SERVICE TECH, THIS WILL NOT BE A NOVICE DO IT YOURSELF JOB. MICROWAVES ARE VERY DANGEROUS, AND CAN CAUSE SERIOUS INJURY. I RECOMMEND TO CALL FOR SERVICE, IF YOU ARE UNCOMFORTABLE WITH HANDLING THE CAPACITOR, OR MAGNETRON.