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?

Well, unplug the lamp driver plug 1J7. Then measure to see if you have continuity between pin 3 and 4. That won't rule out a board problem, but if you have continuity you know for sure it is a problem with the board, most likely a drip.