Hoover U5750-900 just shuts off
I have a similar bagless model U5757-900 that is shutting off. I disassembled the unit, and what I found is that the motor front bearing had accumulated enough grime so that the bearing locked up. When this happened, the bearing outer shell spun in the plastic housing, causing it to soften, from friction heat. This allowed the motor shaft to become off center from the normal, centered axis of rotation. Once this happened, the rotor inside the motor began to rub against the stator, the fixed part of the motor inside. This loaded down the motor, causing excessive current draw, which caused the overheat switch to activate, shutting down the machine.
In your case, it might not be as bad as in my case.
Remove the dual-cannister dirt cup.
To find out, disassemble the unit lower section: on bottom, remove 4 screws around the rotating brush housing. Remove the brush cover, then remove the brush, then remove the belt. Remove the wide plastic housing behind the brush. Remove the two screws, from the bottom, that retain the upper hood. Remove the hood.
On the back, remove the two screws and hose retainer going from the base to the upright section. This releases the hose from the lower section.
Now press the tilt release, and lay back the upright section. Remove the four screws, two on each side, from the trunnion caps, where the upright section tilts on the lower section. Remove the trunion caps. Remove the upright section from the lower section.
Remove the headlight cover (2 screws).
From the back, remove the single screw retaining the bezel above the headlight area.
Slide the bezel upwards and remove.
Remove the four screws that retain the motor cover clamshell. One is from front, and other 3 from rear.
Open motor clamshell halves to expose motor.
Note orientation/position of the three wires on the motor, and remove those wires. Lift motor out.
To open the motor, gently tap the edges of the impeller housing (aluminum shell) on the edge of the shell, in the little gaps around the shell.
Using a 1/2" socket, remove the nut in the center of the impeller. This is a LEFT HAND NUT. TURN RIGHT TO LOOSEN IT.
Gently wiggle the impeller while pulling until impeller comes off of motor shaft. Use a glove so the impeller doesn't cut your hand.
Remove the large flat washer on the motor shaft. Remove the small sleeve from the motor shaft.
On the brush end of the motor, using a small screwdriver, pry out on the brush holders. Set them aside.
Remove the two screws holding motor together. Seperate the motor front from motor rear. Withdraw the motor rotating part.
Look into the inside of the stator, inspecting for bright, shiny areas where the rotor may be dragging on the stator. If you see these, then the front bearing, in the plastic motor frount, is fried. I don't think Hoover sells this part alone. Also inspect the plastic front bearing retainer to see if there is any melting that has allowed the bearing to become off center.
In any case, if there is scrubbed out places inside the stator, then the motor requires replacement, about $70 for the motor... You might be able to find an entire new vac for less... or, like me, find one on the side of the street where someone has tossed out a unit.
You may luck out and find that the impeller unit is just filled with some trash so air cannot get to the motor for cooling... or you may have a shot motor.