Any blockages will need to be removed before the cutters can be freed up to shred again.
If you are unable to get access to the inner parts of the machine, a local repair store should be able to assist.
Let me know your zip code if you would like me to recommend a repairer in your area.
Have you retrieved all debris from within the shredder?
Do you know if it has been damaged by some heavy debris, e.g. staples or paperclips?