Make sure the thermostat is powered and check that the HomeKit code used is the one displayed on the thermostat (see; How do I retrieve the HomeKit code from my Smarther AC connected thermostat with Netatmo? ;). If this is the case, restart the thermostat by pressing the button at the bottom right for 5 seconds until you hear a beep (see; How do I reset Apple HomeKit on my Smarther AC connected thermostat with Netatmo? ;).