My mom (80+) said her microwave (4 y.o.) doesn't heat. Looked at it yesterday and it lights, turntable revolves, fan runs... everything but heat. No loud buzz, no arc marks inside or no visible component damage. This morning she called and admitted she had metal in it and it arced and then it stopped working.

Would prefer to fix, as I could see how this could happen again and again...

Thanks.