The timer is set. The unit auto-shuts off after the selected duration.
Fix: Press the 'timer' button to cycle through duration options until your desired setting lights blue. Available durations are 15 minutes, 30 minutes, 1 hour, 2 hours, 4 hours, and 8 hours.
If that didn't work: The unit may have run out of oil. The glass nebuliser holds up to 30ml. Check the oil level against the MAX symbol. If low, top up with the same fragrance oil, provided the remaining oil hasn't dried up.
Heads up: After the timer elapses, the light will be red. Press the 'on' button again to restart a new cycle.