I have Hoover Windtunnel Vac Model U5421-9RM. It is DEAD. I plugged it in and nothing is happening. Is there a reset button (and if so, where) or a fuse (if so, where)? AND what other options or trubleshooting areas should I try?

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you may have a bad plug/cord. sometimes the transmission on the motor goes bad. any vac shop can fix it.

Hello,
It sounds like your vacuums belt is stretched and worn. You'll need to replace it. Below is a link to a website that gives instructions with pictures on how to change the belt. You should be able to get a replacement belt at any department store or vacuum retailer.
I hope this helps you.

http://www.info-vacuum-cleaner.com/hoover-vac-belt.php

There could be a problem with the vac, the only way to know is to use an ammeter and test the current draw of the unit. If the vac is ok, the breaker could be failing. Plug the vac into another receptacle on a different breaker and see if it still trips the breaker; if it does, then have the vac checked out

Might be that its not exausting enough heat & stutting its self down..

Try cleaning all the filters & the hepa filter..
This will increase air flow & maybe fix the issue..

Hope this helps !!
rate me good ;o)

No belt- it is motor driven. what happens to these if the rollers get stuck(by- say, a Lego® or something)  is the roller itself has an insert which will break, requiring roller replacement. 
simple fix, but costs a little more than a belt. remove two screws holding the roller cover, accessible by flipping the vac over. then figure out which roller is broken, and remove the 2 outer screws on that side. pull the outer bearing cup assembly straight out to remove the rollers.
I have found the rollers as cheap as $3.50 ea/ for generic replacements. Also I reckon any Vac shop would have them on hand.

Have you tried to plug the vaccuum in another outlet that is on another breaker? It could be the vaccuum tripped the breaker. Turn the breaker back on. The manual mentions the thermal protector could have been activated. If this happened it resets and your vac should run after 30 minutes. I have a Widepath Tempo and it has tripped the surge protector. If this is the problem try turning some things off that are also on that breaker/outlet, or unplug them when you use the vac.

if you look below the headlamp of your vac you will find a cable that attach's to a piece called the actuator arm. either this piece is likely broke or the cable itself. always work the power drive smoothly or things will break. also could be that the smooth belt is stretched and will cause the same problem. belts should be replaced every 4 months. good news is all parts above should be under $5. dont let anyone tell you that you need a power drive assy, that usually isnt the case. good luck!!!

If the belt is getting hot, then it is slipping instead of turning the brush roll. Check thge brush roll, make sure the ends are turning freely. Is the motor running ok? If it is then either the brush roll is bad, need a new belt, or something is catching the belt, not letting it move.