I will admit that I've experienced a similar scenario. . . I was about to tear into boards, wires, switches, etc. until it occurred to me that I actually had to have credits registered before I could push the button and start a game. This was after I had done some other work on the game and did not realize the credits had zeroed out.

I'm sure you are aware of that. . . but just in case!

PS -- You can A. Have your game set in free play mode OR B. Trip the microswitches below the coin mech area inside the coin door OR C. Drop a quarter or two in.