The belt may look fine but it still might be stretched and therefore the brush won't turn against the resistance of the carpet. You can get the replacement belts here:

http://vacbelts.com/rainbow-power-nozzle-belt.htm

If the motor is NOT running - press the red button on the nozzle (assuming you have the model with the button on the nozzle)
If it stops working as soon as you release the button - you need a new circuit breaker. If Motor in the nozzle works but brush does not spin - you have to replace the belt.

Good Luck