I assume that you cannot view the menus on the screen either
It may be either a loose connection to the LCD screen, that the LCD screen is faulty and needs to be replaced or worst case that there is a fault on the motherboard.
Try shining a torch at an angle close to the LCD screen to check if you can detect an image at all when using the camera. It will be very faint so try it in a darkened room if possible.
If you can detect an image it means that there is a backlight problem with the screen. Either there is no power being supplied from the motherboard to the backlight circuit in the LCD screen or the backlight circuit in the screen is faulty and the screen will have to be replaced as it cannot be repaired. Hopefully it is not a motherboard problem in supplying backlight power to the screen.
If you cannot detect an image (or even if you can) then unfortunately without a schematic, the only way to determine if it is a problem with the screen or the motherboard is to replace one or the other. Again hopefully it is not a motherboard problem.
Here’s a link to a video that shows how to replace the LCD screen + digitizer in the camera. This will also help you to check if the LCD video flex cable connection is OK.
Here’s a link to the GoPro Hero5 Black Repair guides which may also help if the video isn’t clear enough at some points. Unfortunately there is no ifixit guide showing the LCD screen removal.
Here’s a link to a replacement GoPro Hero5 Black Rear LCD and Digitizer part from ifixit.
Here’s a link to a replacement GoPro Hero5 Black Motherboard part from ifixit just to give a comparison of the cost difference between the two parts.
You may find the parts cheaper elsewhere. Just search online.
