Make sure you got all the connectors back in place correctly when you put the head on. Make sure all the connectors are seated tightly. When I first get an EM I usually clean the connectors with Scotchbrite or 320 grit sand paper just to get rid of corrosion.
Reach in and manually kick the ball to the shooter lane so it runs over the switch in the trough. Does the game start now? Flippers work? Hear a click in the backbox as the ball passes over the switch?