Won t shut off

Blow out the sensors with some compressed air, Using a can of compressed air, look for the sensors that detect the paper and give them a blast or two, then try it, it may take a couple of tries.
This may also be a good time to lubricate the cutting heads, if it is a good machine with metal heads a light oil applied to paper then run through the shredder will pass the oil onto the heads.
A shredder oil or light oil like 3 in 1, these usually have a narrow nozzle to limit the oil flow, draw lines, do a picture or spell your name on the paper with oil and shred it. You have just serviced your shredder.
If it is a less expensive shredder with plastic heads, shredding a couple if sheets of waxed paper will pass the wax onto the cutting heads reducing friction.

Hope this helps.
Ed