There was no power on the cables connected directly to the GPUs.
By the way: It’s very important to connect not only the Riser cards to the Power Supply, also each Graphic card has to be connected separate to the Power Supply 
Have you installed the graphic drivers in your operating system?
And the latest BIOS update?
Also change in the BIOS ‘PEG0’ and ‘PEG1’ values to ‘Gen1’ - if it’s not, it causes a black screen too.
Try to connect from another device witth e.g. TeamViewer - perhaps you can connect and see an error message.