I encounter this complaint all the time. "Everything seems to be working fine, except it's not heating." The clock counts down the time, the lights are on, the fans work.
The main component that heats the food in your microwave is the Magnetron. That is what it looks like on the right of the window. When this stops working, you know. NO HEAT. No Popcorn. That sucks.
Below is a video on how to repair your microwave if it is completely not working. Thanks for watching.