Ryan,
Is there still a sticker on the monitor that identifies it? It could be an Electrohome GO5-801 or an 802 (it could also be a Wells-Gardner if somebody replaced it, but not likely).
One or two possibilities, first is a deflection problem with the monitor. Usually it's caused by failed deflection transistor(s) for the monitor deflection board or bad solder joints. Next would be a game board problem.
Does the game coin up and make sounds like it's playing? Can you put the game into test mode and listen for beeps?
If you can post back with a little more info we can probably help you to narrow it down a little more.
D