In most cases problems with electrical and/or electronic parts are not visible and parts can be tested with a special tools (like a multitester) only. Guessing is one of most inefficient and much more expensive way to fix appliances and a microwave is not the simplest one.

I can only try to guide you through the troubleshooting process if you feel ready to do it.

In order to perform the troubleshooting the microwave (I hope the microwave only) has to be removed from the cabinet. The wiring diagram should be inside the microwave if someone else did not remove it earlier.

Gene.