Sounds like it is probably a ball issue, but just to be sure, have you coined it up?
Make sure all 3 balls are in there. You can take the apron off (the metal V-shape at the bottom of the pin) if you need to.
Chances are, either you don't have enough balls in the machine (easy fix) or the switches/etc aren't working (not hard, but not as easy as adding a ball).
If you have an extra ball, just drop it in. It should automatically 'click' and add the ball if you were missing a ball. If not, it is probably the trough switches.
There is a very, very rare chance your start button is missing a wire or not working, but I've never seen this happen, personally.