Diagnosis
A communication failure between the thermostat and the Honeywell cloud servers, despite local operation. The thermostat's Wi-Fi module may have lost its upstream connection.
How to Fix It
First, check the thermostat's connection status. On the device, press Menu > Wi-Fi. The icon should be solid, not flashing. If flashing, it has lost the local network. Follow reconnection steps. If the icon is solid, the issue is upstream. Restart your home router and modem. Wait 5 minutes for full reboot. Then, on the thermostat, go to Menu > Wi-Fi > Disconnect. Wait 30 seconds, then select Connect and re-enter your credentials to force a fresh cloud handshake.
If That Didn't Work
There may be a service outage with Honeywell's Total Connect Comfort service. Check the status page on the Honeywell Residential website. If services are operational, a factory reset of the thermostat may be required (Menu > Preferences > Factory Reset). You will need to reconfigure all settings.
Heads Up
After reconnecting, allow up to 15 minutes for the thermostat to fully synchronize with the cloud servers and appear online in the app. Do not repeatedly disconnect/reconnect during this period.