What a way to learn why we pull the power adapter and remove the battery during such work. Try another motherboard.
I have a laptop with a damaged display. I would normally use it by connecting it to a desktop monitor with a VGA cable. But the other day i took a display from another laptop and connected it to see if it would work. I know it wasnt so smart but i connected the display while the laptop was powered on. This caused the laptop to cut off completely and now it wont turn back on. There are no lights or anything happening when i press the power button.It's completely dead.

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