It sounds like the wiper drive arm has slipped on the motor. You need to remove the plastic trim below the windscreen so you see the wiper motor. Then you need to loosen the pinch nut on the drive spindle and loosen the drive arm. Then switch the wipers on and off until the motor stops in its park position then move the wipers until they are in the correct position re tighten the nut put trim back.Job done.
Raz, January 2011