The most common cause for the atomiser failing to operate after a prolonged time is the reservoir has dried up and the atomiser needs cleaning. To do this, simply run a damp cloth over the silver atomiser within the reservoir and ensure its free from any salt/debrief which has dried to it. Once done replace the water bottles and ensure the reservoir has around 20mm of water in it. If the reservoir does not have enough water the atomiser will also not work. 

Turn on the atomiser and the unit should spring back in to life.