DIAGNOSIS The unit is likely in 'Child Lock' mode. It's an easy setting to activate by accident—holding the button down for a few seconds locks all controls, and the power indicator light stays solidly lit as a reminder.
FIX To unlock it, you just need to hold the same button down again. Press and hold the power/light button on the top panel. Keep holding it. After about 3 to 5 seconds, you'll see the light blink off and then come back on. Once it comes back on, you can let go. The lock is now disengaged, and a single tap should turn the light and the unit off normally.
IF THAT DIDN'T WORK Try a full reset. Unplug the purifier from the wall for a full 60 seconds. Plug it back in. If the light is still stuck on solidly after that, there might be a fault in the control panel. That's a call to customer support.
HEADS UP When Child Lock is on, the fan speed buttons won't respond either. If you're pressing those and nothing's happening, it's another clue you're in lock mode, not that the whole unit is broken.