You can fix a microwave that won't heat by identifying the core issue based on the situation and the circumstances of the initial problem. This usually takes less than an hour and involves checking basic functionality, switch efficacy and the electrical capacitor.
Unplug the microwave, and discharge its capacitor
Before touching anything, especially the internal parts of your microwave, see to it that it is unplugged. The capacitor contains certain amounts of voltage, which can hurt you even if it is not plugged in. Thus, ensure that it is discharged.
Inspect for any traces of damages or burns
Check the outlet where you plug your microwave. There might be a problem with your power source that causes it not to heat. Examine the cords to see if there are any wires damaged. Fuses that seem to be blown out must be replaced.
Check if the door switches are working
You can do this simply by finding out where the door switches are and removing the leads afterwards. Once you have probed its terminals, check the reading. It should be at infinity if the door is opened and at zero if it is closed. If it is not working, replace this microwave part.