I think you need to provide more info.
Do you have the Extension, have you completed the online registration of that? is the Stat stand alone and hard wired, if so check you have wired up to the correct connections in the stat.
I had a similar problem and it turned out to be the Thermostat that had lost connection with the bridge.
I reattached it by pairing it with the bridge again and control resumed. There is an option in your app to set the device “Bridge” to pairring mode and then just press the thermostat button for a few seconds until the “link” symbol appears and then monitor in the app. When connected try a manual control on the device first (on/off) and then in the app to check.
If this doesn’t work then chances are it is a wiring issue, did you receive a wiring diagram for the thermostat from Tado?
Hooe this helps?