A pretty important part of an EM startup sequence is the score reels. What are they doing? What numbers do they show?
If they are not moving at all, and show anything besides zeros, and given that it worked yesterday, a good place to start would be the connectors between the playfield and the motor board, and the playfield and the head.
And speaking of good places to start, ESPECIALLY if this is your first EM, would be to go here http://www.pinrepair.com/em/ and go to the bottom where the links are to Parts 1, 2, and 3, and settle down in your LazyBoy, and read every word. When you're done, go back to the top of the first page and send a donation to Mr. Harrell.