P2415Q, no display over DVI, HDMI, no menu

Reason Line: Dell P2415Q. After one month working perfectly over display port, no display on screen over Display Port or HDMI, no response to menu buttons, backlight and power LED on but no image on screen.

Problem Description

The monitor worked out of the box for one month using the display port to mini display port cable provided with the monitor, which was plugged into my XFX 7970 and 7950 graphics cards. It supported 4K at 60Hz and was exceptional for photos, videos and games. On Wednesday the monitor stopped working, no image or menu is displayed on the monitor. I plugged in my old monitor to help diagnose via DVI from the same graphics card. When I press the power button on the Dell P2415Q monitor, the power LED lights up and the computer detects the monitor (as shown in the windows screen resolution menu). It sets it as a secondary display but the monitor remains black, I can see the power LED and backlight is on. No power saving mode popup appears on the monitor, when I press the menu buttons there is no response from the screen, it remains black. I repeated with a HDMI cable the same issue occurs with no response or image. I repeated and disabled crossfire (which allows the graphics cards to function independently) and tried the second graphics card with no response. I tested the HDMI cable I used on my old LG monitor, it works well. I have tried connecting a laptop over HDMI to the monitor too, with no response from the monitor. I have exhausted all the tests I can think of, I presume I will have to return the monitor for repair or replacement under warranty. A question to the dell rep and community - is there anything else I can try / do to get it working? 

Monitor / Computer Details

Monitor service tag:
Computer: Non-Dell, self build about 2 years ago. Ivy Bridge i5, Gigabyte Sniper Motherboard, 16GB RAM, XFX 7970 GPU and XFGX 7950 GPU in crossfire, 256GB SSD, 860W Corsair AX power supply (860W on 12V rails). Windows 7, 64-bit. (Secondary Monitor LG D2342).
Operating System: Windows 7, 64 Bit.
Video Card: XFX 7970 and XFX 7950 in Crossfire (tested separately, monitor produces fault with both).
Video Card Driver: 14.12 AMD Catalyst Driver (rolled back to older driver, monitor still produces fault).
Video Card Output Ports: Tested with miniDP and HDMI on both cards, with crossfire enabled and disabled. Monitor still produces fault.
Video Card Cable: miniDP to DP cable provided with monitor. HDMI cable, tested with alternate monitor to confirm it functions correctly.
Monitor Menu Button: Non responsive, when power is turned on the computer successfully detects the Dell P2415Q monitor, the backlight turns on but no image appear on the screen. Pressing the menu buttons yields no result - menu does not appear on the screen. Power LED is on.
Monitor Purchase: Purchased from Overclockers.co.uk March 2015.