The unit fell over, hitting the ground hard and immediately stopped working. Nothing rattled inside like it was broken or loose. I made sure the filter was installed correctly and the back cover was clicked into place, and still no response. Neither the light or fan would engage. I removed the back cover and filter. There are two safety switches, one for the filter at the base of the unit, and one for the back cover at the top where it clicks into place. I turned the unit off at the dial and unplugged it. I placed an insulated slot (flathead) screwdriver to engage the cover switch at the top, then plugged it in. No sparks/fire. Then I pressed the filter safety switch in with my finger, and turned the dial on the top. Instantly the fan and light worked. Apparently when the unit was knocked over, the filter's safety switch was damaged and needs additional force than the filter alone. Blue painter's tape over the filter switch keeps it locked down, and now the unit works perfectly again.