Find out why. These shredders usually have a little sensing lever in the feed throat that switches on the motor when you feed a sheet in. If the little tab has got broken the shredder won't start.
Check and look. You might strike lucky and be able to mend, otherwise it's new shredder time.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix document shredders.