My Hoover Windtunnel Mach 6.9 brush roller quit working. Upon investigating I found that the idler arm pulley was broken so I managed to replace that. Before putting the cover back on I see the control knob for the roller brush moves a plastic piece left or right to activate/deactivate the brush roller. I don't know where this plastic piece connects. It appears to me that the idler are when raised creates tension on the belt which drives the roller. I'm at a loss trying to figure out how that plastic sliding piece raises or lowers the idler arm. Please HELP! thanks in advance.

SOURCE: Hoover WindTunnel Canister: Roller brush motor not turning

These motors have a fuse incorporated in the field winding and it sounds like that blew. These motors sometimes overheat at the front bearing and the bearing will melt into the plastic housing. This will cause the fuse to blow. It sounds like that's your problem. You can remove the motor by removing the three screws on the bottom of the powerhead and then check the front bearing for any bubbling or blistering in the plastic housing. The shaft of the motor should turn easily.

SOURCE: Hoover Wind tunnel V2

Either the brush rolls are bad, ends are bad, or the gear housing/motor asm. is bad and will have to be replaced.

SOURCE: Actuator arm for Hoover U6473-900 vacuum's drive broke

I just changed the actuator arm on my girlfriend's 6437-900 and it's pretty straight forward. Not certain how you got the transmission out, but here's what I did:
-unplug the vacuum
-remove the 4 screws and the plate over the roller brush as well as the roller brush
-remove the two screws and the plastic cover over the end of the hose on the right side.
-remove the 2 screws down in the bottom of the housing on either side of the drive wheels
-remove the single screw in the middle/bottom of the housing
-push the two tabs on the inside of the outer edge of the housing to remove the top cover
-flip the vacuum over, you'll see an "e" clip holding the actuator arm in place being careful not to lose it
-use a torx bit to remove the old cable hold down
-gently bend the metal piece over the arm back far enough to remove the old arm. Note the slot in the metal piece, you'll need to make sure you get the metal tab in the slot when you reassemble everything.
-put the new arm in, about $8 at a Hoover repair store
-put the metal piece over the rod that the e-clip fits into, making certain the tab and slot are aligned
-press down on the metal piece once it's on the rod with the tip of your screwdriver so it snaps in place, carefully put the e-clip back in it's slot on the rod.
-put the cover back on, lay the vacuum flat, put the cable end in it's channel on the arm, tighten the hold down.
-flip the vacuum over, plug it in and turn it on, being careful to keep hands, fingers, ties, hair, small children, pets, etc away from the spinning roller brush
-move the handle, still in the off position, up and down to make certain the drive wheels don't move. If they do then readjust the position of the cable on the end of the arm. The wheels should not move at all with the handle in the off position.
-enjoy

SOURCE: auto brush on hoover windtunnel doesn't spin anymore

I have a Windtunnel S3765 with the same problem - and it has very little use so I'm confident it isn't wear. I find that the wheels do not raise the agitator brush sufficiently high to reduce the drag caused by excessive contact with the carpet. If I raise the front a bit the brush spins as it should. It also works if I take the housing apart and use it without ..... but this is tricky.

Unfortunately I haven't figured out a way to hold it up efficiently. I am toying with afixing something to the bottom of the housing to raise the front enough to give the brush a little more room.

SOURCE: Hoover WindTunnel Self-Propelled V Belt

when you released the end to slide the new belt on it allows the tension roller to slide into the wrong position. Remove the top so that you can see the roller, at the left side of the roller mechanism there is a screw, loosen it enough to allow the roller to return to the correct position.
good luck