Check all your molex plugs in the head. Reseat the plugs and ribbon cables on the boards.
If you had the playfield up check the plugs off the transformer and reseat.
And also if you removed plastics and ramps check that any plugs you disconnected got put back in the right spots.
If that fails. With the head open.
Turn on and watch the LEDs on the cpu.
And listen to the beeps.