Guardian AC4100/AC4175W/AC4150 air purifier has no power

1. Remove back panel cover and remove the filter
2. Under where the filter had been sitting there will be a lever/tab sticking out (see picture below).  You should be able to push this down with your finger.

3. Turn the unit and the UV light on, then press and hold this lever/tab down. At this point hopefully the unit will turn on.  If it does, reinsert the filter, making sure it is snug against the switch.  Put panel cover back on.