Not heating The microwave is not heating. All buttons work, light works-it's just not heatong anything.

If the oven is doing everything else properly (light comes on, fan runs, turntable goes around) but there's no heat, you may have a failed magnetron tube (the microwave source). There's also a relay that controls power to the tube that may not be working correctly, or the control circuit board is bad. It's also possible one of the door switches is not working (so the oven thinks the door isn't closed), although this usually keeps the turntable and fan from running.

Microwave oven service is not for do-it-yourselfers. There are potentially deadly voltages present even with power off, and a chance of microwave leakage if repairs aren't done properly. Let a professional give an estimate to repair it, or just replace it if it's under $100. (When I was servicing these things it wasn't unusual for a repair to cost over half the price of a new one. That's one of the reasons so few shops still work on small appliances.)