Leaving it in the Auto position shouldn't have damaged it, unless it has an electrical defect.
What usually causes a paper shredder to stop working is shredding too many things at once, stressing out the motor. OR shredding too many things, one after another after another, overheating the motor.
Another hint to keep a shredder healthy is to lubricate it occasionally. They make products especially for this, but they are kind of pricey. I just spray WD-40 on a sheet of paper and run it through the shredder once a month.