1. Remove the access plate (a plastic frame over the brush roll) and check the brush and belt for damage. Be sure to clean off any debris on the brush. If brush will not turn by hand, you’ll need to replace the brush. You can do so by clicking here: http://www.bissell.com/parts/?sc=part&productid=2432&partid=4846
If the belt is broken, you can buy extras by clicking here: http://www.bissell.com/parts/?sc=part&productid=2432&partid=4680
2. If the brush turns on hard floors, but not on carpet, adjust the height with the knob on the base. Using the unit with the height adjustment set too low may cause the belt to stretch. If the belt is stretched, the brush will turn on hard floors, but will stop when it comes into contact with carpet. Try a higher setting. You may need to replace the belt (see above link).
3. If the brush still won’t turn, please take your vacuum to a technician. You can find your nearest BISSELL® Authorized Service Center by clicking http://www.bissell.com/service-centers/ or by calling 1-800-263-2535, available 24 hours a day.