The brush will not rotate when the unit is locked in the upright position, because an internal micro-switch turns the brush motor off.
Make sure both power switches are turned on.
Make sure the canister is seated properly on the base of the unit.
Remove the access plate and check the brush and belt for damage.
If the brush will not turn by hand, or the belt is broken, replace damaged parts. Clean off any debris on the brush roller.
If the brush still will not turn, please take your machine to a local BISSELL Authorized Service Center for inspection. You can locate a Service Center online at http://www.bissell.com under the Customer Support tab. Enter your zip code in the Service Center search box or by phone, 24 hours a day, by calling 1-800-237-7691 and choosing option #1.