Power off the game.
Examine all of the connectors coming out of the back of the big silver box (computer) under the playfield, use a flashlight if neccessary.
Check to see if any are disconnected or loose (tug on them a little?) Maybe unplug and re-plug them in one by one.
Carefully examine the wiring harness on the playfield itself, from the bottom to the top, gently comb through it with your finger tips, see if anything seems disconnected.