I'd clean the switch with a clean piece of paper. Gently pinch the paper between the contacts if the switch and pull it through. A few passes should be sufficient.
Make sure the switch is adjusted properly, and that the screws holding the switch to the playfield haven't gotten loose. Make sure that the ball rolling over the switch's actuator is enough to push the switch closed.
If the switch has a capacitor wired into it, try cutting one leg of the capacitor off and see if that gets the switch working. Either way, if the switch has a capacitor, I'd change it for good measure. Also, gently tug on the wires going to the switch to be sure they aren't loose or hanging on by a thread.
Are any other switches not working?