It would appear you have stripped the gears or broken a belt. Without more information like the make and model of the mower, I can only give you a general answer.
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Also, check [http://www.kichline.com/chuck/fixit/mowers/packeddrive.jpg|here] and [http://www.kichline.com/chuck/fixit/mowers/lbdrive/lbdrive.html|here] for other possible problems.
It would appear you have stripped the gears or broken a belt. Without more information like the make and model of the mower, I can only give you a general answer.