Sometimes if you turn the ignition off while the wipers are operating, then the wipers will do that thing you are talking about. They will act like they are dead and you won't even hear the wiper motor. To make them work again turn on the ignition, turn on the wiper switch, then move the wiper by hand (bring them back to their original position if they are not there). As soon as you move the wiper they should start operating. This is like waking them up. Don't use force or you may brake something. I do this to my car so far it worked every time when wipers stopped working.
Let us know what if this helped.
1989 230E, 8v, 166.000 km, updated to 94/95 trunk & hood
2002 Daewoo Nexia 50.000 km
1987 VW Jetta GLE 16V, Recaro seats
1982 Volvo 240 DL (lovely car!)
and few more american cars.