DIAGNOSIS The solid red light is a filter replacement indicator. The unit has a runtime-based timer and will stop operation when it decides the filter is 'expired'.
FIX Reset the filter indicator. With the purifier plugged in, press and hold the Filter Reset button (the one with the circular arrow icon) for 3-5 seconds until the red light turns off. The unit should restart on its own. If it doesn't, press the power button.
IF THAT DIDN'T WORK The internal filter timer might be glitched. Unplug the unit from the wall for a full 10 minutes. While unplugged, remove and re-seat the filter to ensure it's properly installed. Plug back in and try the reset procedure again.
HEADS UP This timer is based purely on runtime, not actual filter condition. You can reset it with the same filter if it's still clean. Mark your calendar for a real visual check in 6-8 months.