Microwave seems to operate normally but does not

Test it. Put a bowl of water in the microwave and place a small (1 or 2 sq inches) piece of aluminum fold next to it. Power the microwave on for 2 or 3 seconds. If the foil sparks, the oven is creating microwaves and should be functioning correctly. If nothing happens to the foil, then you have no microwaves being generated by the klystron tube.

You can spend 80 to 150 dollars to have the oven checked and repaired or you can find another on Craig's List in your community for less than 100 dollars. Alternately, you can replace with a new one for whatever you are able to spend (from 95 to 600 dollars).