The turbo brush may just need to be cleaned. If hair or other large debris gets wrapped around the brush, it can prevent it from spinning. Since the top of the turbo brush is clear, you can look in and see if there is a lot of hair wrapped around the brush. If so, you can open it up by removing the two screws on the bottom of the brush and popping the clear cover off, then cleaning all the debris from the brush (scissors work well if its hard to get dog hair off). Be careful to see how the brush goes back together as you take it apart.
If cleaning the brush doesn't fix it, the belt might be broken. As you're looking at the top of the brush, there is a little belt that goes from the brush itself to the large blue/green gear (it's red on some models) so that if you reach your hand in and turn the gear, the brush should spin. If this belt is broken or missing, it will need to be replaced. You can probably find a replacement belt online, or you can order a whole new turbo brush from the Bissell website