Could be a hundred different things on the board. I can't give you any real specifics without seeing a photograph of the screen, or a better description of the problem.
These boards aren't hard to fix. It could be something as simple as a bad socket, or it could be as "complicated" as a bad TTL chip somewhere.
Worse comes to worse, buy another board. What's a Pac board go for these days? $50? $75? Don't know, but they're real common. As others have said, the value in this game is the cabinet. If the cabinet is in good condition, the electronics are easy, easy, easy.
-Ian