Resetting a wiper motor can sometimes become necessary if the wiper arm was frozen while extended or if it has been clogged with leaves. This causes the wiper motor to "forget" where it is and may think it is down when really it is halfway through a swipe. It can be easily reset by hand, but all the debris will have to be removed to make it possible. There is a reset electrical contact that will switch the ground in the motor, thus causing it to reset automatically. This applies to the wipers on the front and in the back, for those vehicles with a rear wiper.
Put your vehicle in park but do not turn the engine off. This is one of the few times when working on a running automobile is OK. Turn the heat all the way up in the vehicle and turn on the defroster to help with the ice removal. If you are just clearing leaves, then you do not need the defroster on. Leave the windshield wipers off for now.
Chip away at the ice with an ice scraper. Clear all the ice from your windshield and get into the crevice that the wiper motor rests in and clear out the ice. You may need a small ice pick to completely remove the ice. If there are leaves or other debris clogging the area, then use a pair of pliers to get down into the crevices and remove the debris.
Put on a pair of gloves and slip your hand under the wiper arm. Pull the arm down towards the fully extended position, which will be on the right or left side depending upon the vehicle. Do not force the blade past its stop, just make it come into contact and then pull it down to the resting position on the opposite side of the windshield. If the arm stops before the full extension, then you quite likely have more debris to remove. Remove the remaining debris with the pliers or by hand and restart this step.
Turn the windshield wipers on at the lowest setting and see if it moves. If it does not move then there could be an electrical problem preventing the wiper motor from resetting, which will require a mechanic with diagnostic equipment to fix.