I have a bagless hoover windtunnel. The plastic flap down by the roller wont open! thus nothing will **** into the filter

SOURCE: hoover s3755 bagless windtunnel canister

Very likely it's the cord at the nozzle neck. These cords flex when you vacuum and they often break internally where the cord goes into the neck. Turn the vacuum on and wiggle the cord at the powerhead and see if the powerhead comes on and off. If this happens, then you will have to replace the cord.
On some of the early models, the receptacle in the hose, where it plugs into the top of the wands, will break away from the plastic pins it sits in, and the receptacle will slide back into the handle, which means that it will not plug into the powerhead connection when you connect the hose. You can check this by looking at the end of the hose and observing if the receptacle is sitting flush with the opening it sits in. If it is pushed back, it will not make a complete connection with the powerhead cord that is sitting at the top of the wands.

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

SOURCE: Hoover Windtunnel shuts off

I found the following solution on this website. I have searched for hours and finally found this solution, it works! I HAD the same problem after cleaning the filter mentioned below, it is now working like it was new.

***by maverick_bea***
yes this machine has a thermal protector in it to shut off the machine if it gets to hot, this is normal. if there is no clogs in the hoses or in the machine then you will need to wash the filter at the top of the machine. you will have to remove the dirt cup and at the top you should see a half moon orange, well inside of this piece is the washable filter, clean and let it dry completly before putting back into machine. if this gets dirty and has not been cleaned, you can lose alot of suction. in some cases if this filter is not cleaned it can cause the machine to overheat so bad that it can damage the motor.

SOURCE: Hoover U5780-900 Windtunnel Cyclonic Bagless

You probably need to change the belts. Here are the belts you need and the manual:

Hoover 5780-900 Belts

Manual

Testimonial: "Replaced the belts-- works GREAT!!! Thanks"