There may not be water in the tank. Check water level and fill accordingly.
For models with a Humidistat, the set RH% has been reached. When the set RH% is reached, the unit will stop producing mist. The humidifier will automatically produce mist again to maintain the RH%. If you would like the humidifier to continue to produce mist, either increase the RH% or continue to press the HUMIDITY button until "Co" flashes on the LCD screen and AUTO is no longer indicated. You can adjust the mist using the MAX/MIN button.