DIAGNOSIS The filter reset sensor wasn't triggered. The unit has a mechanical switch behind the filter door that must be pressed.
FIX Unplug the unit. Open the front filter door. Firmly press the small white plastic reset button located on the right side of the interior frame. Hold it for 3 seconds. Close the door and plug it back in. The light should reset.
IF THAT DIDN'T WORK Check that the plastic tab on the filter door itself isn't broken. That tab depresses the reset button when the door closes. If it's broken, the door won't press the button.
HEADS UP Never use the reset button to try and extend filter life beyond the actual replacement interval. It's for resetting the timer after a genuine filter change only.