More than likely your problem lies within the long interconnect connector that runs between the CPU and the MPU. Try giving everything a little reseat and wiggle to see if the game jumps back to life. If it does, you'll know you'll need to replace those connectors. It's not terribly difficult.

Alternatively, gently pushing on the socketed ICs on the MPU might bring the game back to life. William's used Scanbe/RN sockets that were just awful quality back then. If doing so works, you'll really want to swap them out for proper sockets.

At the end of the day, you'll probably want to resocket everything anyway. That's the only way to really bulletproof things.