I destroyed my Main Board inserting a cartridge with the power on. My machine went completely dark and I needed a new board.
Start by removing all cartridges and see if the conditions change. Check to see if the Main Board has two red LEDs lit. That indicates the main processor is somewhat sane. You could have a connector that is only partially seated. I would start with the connector on the back of the display. If you are adventurous, Take out the five screws that secure the base plate exposing the Main Board. Re-seat all connectors on the Main Board. Do all of these things with the power off.
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