@GnGeorgiou 

Check whether you can see the camera when your phone connects to the same local wifi network as the camera. If the camera has disconnected from cloud, this thing might happen if you're on a remote network (e.g. celluar data or other wifis.).

Long-press the power button for 3s to turn on the camera, what's the led status of the camera? If it turns solid amber, it is indeed disconnected from the cloud. For this case, you may refer to the suggestions here in case 2 for troubleshooting. Cannot use the Tapo app to control my smart devices