I know that on the iPad 2 it was common that people would unplug the LCD connector using something metal like a pry tool of screwdriver to flip the clip holding the LCD connector in place. It would often short out the board and kill video completely. Only way I know of fixing it to replace the motherboard. Which basically is to replace the iPad.
Sounds like the same thing. I've seen it before. If you tried to hard reset a time or two and it didnt work. Check those connections one last time but you may have shorted out the board and killed it.
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Hope this helps!