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If the microwave still turns on and off but it just doesn't produce any heat, then unfortunately the magnetron has shorted out from the staple as you suspected. The magnetron is the part that actually produces the microwaves and it'll arc out and can fail if metal is inside the microwave when it's heating. The part is usually about $150 and costs another $150 if you need someone to replace it for you so it may not be worth fixing but that is what you're looking at as far as costs.
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Hello again. I'm just following up because I haven't heard back from you and I see this question is still open and unrated. Is there anything else I can help you with?