Make sure the thermostat is properly powered and check that the HomeKit code used is the one on the thermostat (see "How to recover the HomeKit code of my Smarther with Netatmo connected thermostat?"). If this is the case, restart the thermostat by pressing the button in the lower right corner for 5 seconds until you hear a beep (see "How to reset Apple HomeKit on my Smarther with Netatmo connected thermostat?").