OK, first make sure you have credits (should show in the credit window).

Now go into switch test mode.  Make sure all targets are up and the ball removed.  There should be a flashing '0' in the Ball in Play window, if not you have a stuck switch, which you will need to identify from the manual and fix.  If the switch does work in test mode, and you have credits, you most likely have a CPU problem (unlikely).

Once you have the flashing '0', push and hold the credit button in - if the switch is working, the appropriate switch number should show up in the window.  If it does not, look for the following:  broken wires from the credit switch (check each wire colour to other switches on the coin door - a break anywhere can be the problem, eg someone removing a coin switch wire and separating the two wires); credit switch contacts not meeting (visual check); poor connection for the door connector just inside the cab; poor connection on the CPU (one of the right hand connectors - follow the wire colours); broken/cold solder joints on the CPU for the switch wires.

Hope this helps.

Ian