The switch you are looking for is located on the score unit itself, the one that keeps going around and around. If you look at the lower left area of the picture you posted, you can see a switch stack mounted just behind the plastic reel with the numbers on it. Look just to the right and below the upside-down "6" and you will see a small switch stack mounted to the top of the bracket that holds the drive coil onto the frame of the unit. That is the switch stack that contains the zero switch you need to check.
There are three switches on this stack. One is open all the time, and closes at the 9th position to cause a carry to the next higher score unit when going from 9 to 0 (but this switch may not be on the 10,000 unit). The other two switches are only open at 0 and are closed in all other positions. One of these shuts off the pulses to its own score unit when it gets to zero. The other one keeps the game in reset until all score units in the machine are back to zero.
It sounds like what is happening is that the zero switch for the running score unit is never opening, so it just keeps going around and doesn't stop at zero. This could keep the game in reset forever. Also, if the other switch that is supposed to open at zero never opens, that would also keep the game in reset even if all score units return to zero and are no longer moving.