Why can't I twist the two main parts back together to lock them in place?

Part 26

https://evacuumstore.com/s-1587-u6425-900.aspx

https://www.google.com/search?q=Hoover+wind+tunnel+parts+schematic

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I took off the dust canister and then the 2 Phillips screws on top of the rewinder. I was searching how to get at the rewinder so I took out the HEPA filter and holder. I then pressed down on the rewind leaver and it rewound. I am not sure why but maybe what I removed was putting some pressure on the rewind mechanism or I just had good luck.

If you turn the vacuum over there will be (4) screws that hold on the base cover for the roller brush. Remove the screws and the cover and you can get to the belt(s). You can get replacement belts here:

Hoover Windtunnel Belts

Hello my good friend, this cleaner doesn't have a reset button or anything, it resets by itself after cooling down, it takes about 30 minutes to cools down.

Place handle in low position and turn cleaner over so bottom side is up.
Remove the 4 screws in bottom plate and lift off.
Look on the left side of the agitator brush. You should see the belt there. Remove agitator brush roll and used belt.
Discard belt.
With lettering on outside of belt, slide new belt over motor pulley.
Insert agitator though belt and place belt in belt guide on agitator.
Grasp agitator firmly and fit it onto position by first sliding end opposite the belt into the rectangular slot on side of cleaner.Pull other side of agitator into place
Turn agitator making sure belt is not pinched between agitator and agitator shield.
If this does not take care of your problem, then check your filters for dirt and your entire vacuum for blockages. Both filters do come out. If you can't get a new filter right away you can gently clean the paper filter and remove a lot of dirt. (I recommend outside)! The other filter is washable.
Make sure it is well dried before replacing.
Good luck.

the hoover website does NOT give screw sizes,unfortunately!

cut and paste following info
http://www.fixya.com/support/p40544-hoover_ultra_u6430_900_windtunnel_bagged/manual-4481/page-13

Need a manual for Hoover Vacuum Cleaner, Wind Tunnel Model:u6630900

Check the inlet down by the brush roll to make sure it isn't clogged. If not and the hose isn't clogged, then probably the thermo protector (part #28382093) maybe bad. It is mounted near the motor. Could be motor Part #43576188. But I would suspect a clog in the inlet or hose, or the thermo protector has gone bad.

what probably happened is you swapped end over end when you replaced the beater brush. anyway go to thehooverstore, find your machine and you'll see an exloded diagram of your machine.