I have a small Ridgid 4 gallon wet/dry shop vac (model WD4050). I was cleaning the filter and when I replaced it, the vac didn’t work anymore. When I turned it on, it’d make a loud noise and emit a burning smell. I remembered that when I turned the unit over to clean the filter, I heard a “klunk” sound. That sound was the “float” falling down the float cage. The float keeps water from going into the motor. I guess the motor was so hot that when I turned the vac right-side-up the float never fell back down again. It had been fused to the motor (see the melted plastic on top of the float). With a slight pull, I pried the float off of the motor, put everything back together, and it now works fine.