Sometimes it will happen, when you repeatedly minimize and go back into your app while you are videocalling, that your own camera screen turns and stays black. The app may no longer connect to the camera on your phone.
To solve this, you need to restart the app entirely. Locking your screen, or minimizing the app won't be enough.
If that does not solve things, you will need to restart your phone. This should release your camera so it can be contacted by the app again.
If that does not sovle things, please contact us.