Need Appliance Repair Help? Ask a Technician for Answers ASAP
If the unit is less than a year old, I'd call warranty and they will come out and fix the issue at no charge. I would not mention you used styrofoam in the unit, but unfortunately it sounds like your transformer or your magnetron overheated due to the styrofoam causing the microwaves to react to strongly so either the transformer or magnetron will need to be replaced and that's about a $300 repair if it wasn't under warranty.
We are on the honor system here. Please remember to rate my service by selecting a star at the top of the screen before you leave today and reply back here letting me know with a rating of 1-5 stars (5 being the best). If you need anything else, just let me know. You can continue asking follow-up questions at no additional charge after rating my service if you still need assistance.