First, make sure your thermostat is properly powered. Then check that the Wi-Fi is working.
If you have changed your router's Wi-Fi password or installed a Wi-Fi repeater, you will need to reconfigure the system's Wi-Fi by accessing the Home + Control app settings, selecting “Manage my home,” selecting the room where the thermostat is installed, selecting the thermostat, and selecting “Wi-Fi configuration.” Then follow the instructions provided by the application step by step.
Updated