Likely the switch in the outhole that is supposed to detect whether the ball drains. Look at the switch matrix and check the wires on both that switch and all others in the same row and column to look for a broken or missing wire. Note that there could be a broken wire on another switch in the column or row that is affecting the outhole switch but not others.

Report back when you do. If it is not that, there are a few other things to check for.

Well, there are actually two outhole switches because it's a 2-ball game. Did you have both balls loaded? The game will never start or serve a ball unless both balls are there and both switches are activated.