Hi. could you provide me the brand and model of your shredder so that i can provide you the right troubleshooting. Though i have some inputs in your problem which is very common to all shredders. please try this one.

This may be a mechanical or electronic sensor located in the center of the paper feed slot. If it is electronic it may just need cleaning(qtip dipped in rubbing alchohol. If it is a mechanical sensor check to make sure it has not been broken or it may be stuck because of paper that has become wedged behind it(you may need to jiggle the switch a few times to free it). If none of this works you may have a burned out switch or a bad pc board, contact manufacturer for more info.