DIAGNOSIS Loss of communication between the thermostat and the Honeywell cloud servers. This is usually a local network issue.
FIX First, reboot your home router. Wait two minutes. Then, at the thermostat, go to Settings > Wi-Fi. Check that it shows 'Connected'. If not, reconnect it. If it shows connected, go to Settings > Wi-Fi > Advanced > Reset Wi-Fi. Re-enter your network credentials to force a fresh connection.
IF THAT DIDN'T WORK The thermostat may need a reboot. Pull it off the wall plate for 30 seconds. Reattach it. Wait 5 minutes for it to fully reconnect. Check the app again.
HEADS UP The thermostat and app do not communicate directly. They talk through Honeywell's servers. An internet outage or server problem on Honeywell's end will also cause this, and you must wait for it to resolve.