This has worked fine up until today.
The first screen works absolutely fine. The second screen however, is seen by the OS, is visible in the display settings and works absolutely fine in mirror mode.
However, when I disable mirror mode, the second screen just displays black, although the mouse cursor is visible on in. The screen is also recognised by the desktop manager as a valid desktop; it allows windows to be moved onto it (which are not visible beneath the black overlay) and if I use the workspace switcher I can see a rendering of what is on the screen. If I aim the mouse correctly I can even click windows on the second screen and move them back to the first.
This happens on Unity, Unity 2, GNOME and GNOME without effects.
The events that led up to this are as follows:
I've been having problems with Chrome recently, it flickers a lot (more or less this issue - Google Chrome "Flickering") so I just stopped using it (suggested solution didn't work). Before that I did try updating the AMD graphics drivers to the latest ones (card is AMD Radeon HD 7700) which didn't fix it.
Earlier today (before the screen issue) I tried using Unity 2D to see if it would fix it, which is when the second screen just stopped working (as described at the start). I also tried Chrome there and the whole screen had graphical glitches, not just the web page and the computer locked up.
After restarting and going back into normal Unity, the second screen was still not working (as described above). Chrome again brought the whole system down. And the same issue also occurs in GNOME (with and without effects).
Edit: Graphics driver version is amd-catalyst-13.12-linux-x86.x86_64. Also discovered that the sound (over HDMI) is giving glitches (distorted and repeating sections) and am also getting sporadic graphical glitches, particularly when opening the Unity main menu.
Another strange thing I've noticed; the screen that has the black overlay isn't a particular screen, it's whichever is set to be on the right of the other monitor. If I move the layout of the screens in display settings around to other configurations, then some times both screens have black overlays and other times only a part of the second screen has a black overlay.
Intel i7 3.4Ghz (Haswell) XFX Radeon 7700 1TB HDD z87 chipset 32GB DDR3 RAM
I also tried the open source drivers, they didn't recognise the card at all. Ubuntu would get as far as the loading screen and then stop (still displaying the logo and 5 lights). Also tried the beta Catalyst driver, exactly the same symptoms as previously as did reinstalling the original Catalyst driver.