Sometimes you can get away with putting metal in your microwave... But. Your son didn't. So now there are several possibilities damage to the Magnetron and or High voltage transformer and or electronic controls. The Magnetron and transformer are pricey items so repair cost might exceed the replacement cost. If you click on the LG (model number) in the heading at top of page you can scroll down to a service manual you can use in your repair efforts. Good Luck!
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=== Update (06/15/24) ===
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Link to the manual: [document|32483|LCRT2010ST Service manual]
Sometimes you can get away with putting metal in your microwave... But. Your son didn't. So now there are several possibilities damage to the Magnetron and or High voltage transformer and or electronic controls. The Magnetron and transformer are pricey items so repair cost might exceed the replacement cost. If you click on the LG (model number) in the heading at top of page you can scroll down to a service manual you can use in your repair efforts. Good Luck!