recently i obtained a ATX power supply from an old computer of mine to power my RAMPS 1.4 board. I followed a diagram on the reprap wiki showing how to connect the power supply to the board, with the yellow cables (in the + terminal) and black cables (in the - terminal) alternating. I then connected the board to the computer using the given connector. Once everything was connected, I plugged the ATX power supply into the wall socket and turned everything on, and the computer turned off. I immediately disconnected the power from the wall and unplugged the board from the computer. The board was very hot so i let it cool down for a couple of minutes and then connected it back to the computer, except now the RX LED is red and I can't connect to it using pronterface. Did i fry the electronics, or is it something else. The computer is working fine, but i'm not sure about the board. Please help.

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.