There are really two reasons why the self-propelled wouldn't work. It could either be the belts on the vacuum cleaner are stretched or broken. The belts on this model control both the self-propelled transmission, as well as the brush roller. Here's a video that shows you how to inspect the roller brush and belts to make sure that they are installed properly and working. Try this repair first, as this is the easier one to fix.
The other problem if the belts are installed properly that would cause the self-propelled wouldn't work would be the self-propelled actuator arm. There is a cable that starts at the handle of the vacuum, and runs all the way down to the floor head of the vacuum that controls the transmission. As you pull backwards on the handle of the vacuum, the cable pulls this actuator arm backwards, which then turns the wheels backwards, and when you push forward it turns the wheels forward.
The problem with the way it's designed however, is that if you pick up the vacuum cleaner by the handle of the machine rather then using the carrying handle on the back of the vacuum, it causes too much tension on the actuator arm, and will cause the cable to rip right out of the arm.
This is a VERY common problem on this model. To check and see if this part is broken on your vacuum, step on the handle release pedal so that the handle is in the vacuuming position, push the pedal a second time so the machine lays flat on the floor.
Now if you look right behind the height adjustment switch, and the rug / floor switch where the floor part meets the upright part, you should see a cable that connects into the actuator arm. Pull up and down on the handle and look for movement. Is the cable loose with only a nut attached to it, or is it resting in the actuator arm?
Here's a very helpful link that shows you exactly how to replace the actuator arm:How to Replace the Actuator Arm on Hoover Self-Propelled Windtunnel
If this didn't help you, and the problem is still happening, please just use the add a comment form here on Fixya and let me know that you're still having problems. I'd be honored to go through and do some more advanced diagnosing with you.
I hope this we helpful to you. If you did find this helpful, please be so kind and leave feedback for me. This is why repair professionals like myself continue to come back and give free repair advice, to get the recognition of a job well done.
Thanks so much,