I have a Kenmore canister vacuum that has worked well for me but for some reason I can not get the power head to work. It used to work but won't now. I have taken it apart & cleaned it as well as checked the belt. While the cover was loose I plugged it up to see if it would work & it did but soon as I screw the cover back on it will not work. I though it might be the reset button on top but laid it down inside so the cover would not have an affect on it. What could it be & what can I do?