Check that the device is plugged in and the wall socket switched on.
Check that the device is connected to the internet and controllable via the Android or iOS app, if the device is greyed out it is offline.
If the Wi-Fi connection is disconnected (ie the SSID or Wifi Password has changed) then follow the TPLink guide - How do I move my Kasa device to a different wireless network? | TP-Link
If none of these work please send me a debug email via the support tab in the app. Thank you