Most likely the motor has broken down. This could be a result of poor manufacturing, overuse or overloading. For any of the reasons, a stuck cog or belt could have jammed the motor and caused it to burn out. If your shredder is new, try and get it fixed under warranty. If old, you could try taking it to pieces but it may be cheaper to get a new shredder because spare parts can be hard to find.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix document shredders.