Well, it means either something is wrong on the board, or your column wires are shorted together physically. Were you doing anything to this machine other than playing it? Have you looked for any wiring that might be dripped on or smushed? My off the cuff guess is that it's cpu related, but wiring is almost just as likely. If it is indeed on the board, you're probably looking at a bad U53, but if you were doing any repair in that area, it's very easy solder splash two pins together as well. Do you have a multimeter and know how to use it?