Hoover windtunnel bagless vacuum stopped spinning only when it is not on upright position
It is possible that the belt has stretched or worn to the point that it is unable to spin the brush whenever the brush has to push against the carpet. Often in these conditions the belt may very well be able to spin the brush freely when the brush is not trying to push against anything. 

Belts can also get old and hard with a glazed surface that prevents the motor shaft from getting a good grab against the belt. When you install a new belt you may note the difference in the rubbery grab of the new belt to the motor shaft.

So with that said - do try a new belt.