Search the web for "Clay's Guides" (not sure where those are mirrored now, and I have lousy connection myself ATM). "Mark's pinball repair pages" also helped out a few years ago, distilling Clay's into something a little less overwhelming. Pinwiki mugh also have a troubleshooting flow procedure.
Essentially you need to find out why the MPU is not booting up. "Flashing lights", attract mode, displays, etc all come from the program in the Game ROM so if the board won't boot, the ROM program won't be read, therefore you only get GI.
All sorts of reasons this can happen, and Williams games of the era are notorious for lack of any meaningful diagnostics. "your problem" might be relatively simple, or much more difficult.
Clay's Guides are as good a starting point as any. There's not really a one-size walk through given so many potential failure modes, but if you read those resources a time or two, you should be able to start narrowing things down, and other resources might even make more sense too.