I'd unplug the ramps board from the MEGA board and see if you can communicate with the Arduino without anything plugged in. Try uploading a test program from the Arudino IDE such as Blink. If you can't do that, then your microcontroller is likely fried. If you can do that, then that's a good sign. Upload the firmware of your choice with just a basic configuration for your setup. Next connect the ATX power supply to the ramps to just the 5A socket, the one in the lower left corner. Negative (black) should be closest to the corner. Don't plug in anything else, see if things work then. With nothing plugged in, there shouldn't be any significant power usage aside from just powering your Arduino. Is it still getting hot/shutting down the computer? If so, you probably have a short somewhere in the RAMPS board.
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