It is most likely the teeth inside the wiper motor are gone. You can remove it and inspect it easily. If no damage clean the contacts and put it back to solve the problem. If the plastic teeth are broken you have to buy a new wiper motor.
My wipers motor did the same and stopped middle of the rain bomb in Sydney. I had to drive with no wipers for a week. I purchased a new one from amazon and it is working great.
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