Black display on the camera what to do

@andreageorge this "reset camera or restart your phone" sounds like your phone can't find or initialize your camera. Try in safe mode to see if this is caused by a third party app. If it works in safe mode, you will have to remove the 3rd party apps that interfere with your camera. Try this:

While the device is powered on, press and hold down the power key. In the pop-up menu, press the Power key. touch and hold Power off until the Reboot to safe mode message appears. Tap OK to restart in safe mode. From Sony

If it doesn't work in safe mode, you may have to consider replacing the camera. Use these guides Sony Xperia XZ1 to work on your phone.