Shredder keeps running after paper is inserted and

I used a Q-tip with alcohol to clean the optic sensor and it fixed mine.

Look if the input sensor is not stuck.
It can be a small "tongue" that tilts when the paper is inserted - some small piece of paper might block it from coming back to original position.
Another type of sensor is an optical one - inspect the feed area for any dust or paper particles. A good blow with compressed air might help.