I totally did not notice that. Somebody must have pulled it and replaced the wood with a piece with no hole for a door. So I'm guessing there's a credit button somewhere? Note: Don't leave the thing sitting there with credits in it, the attract mode doesn't run with credits and it burns the monitor. Mine had clearly been in somebody's home, because there was bad screen burn saying "Push Start to begin" or whatever the start message is, along with "99 credits"
Also, if you want a coin door, PM me, I've got a few to get rid of.