If you are using an old legacy Honeywell Wifi thermostat (Mine is RTH6580WF) and it just stopped working, don’t look into HA!
All you need is to go to https://mytotalconnectcomfort.com and accept their new privacy agreement which is blocking the HA integration component to connect to Honeywell.
You can confirm if you have the same problem by checking your HA logs and you should see connection errors.
Good Luck,