I'm not sure whether this is a problem with the monitor, the graphics card, or (most likely) the graphics drivers.
I have a Sapphire ATI Radeon HD 5870, which has two DVI outputs. The drivers for these are up-to-date (I downloaded the 10.9 drivers from here). On the DVI outputs I have two identical Iiyama ProLite E2407HDS 24" monitors (both connected with DVI cables) - these did not require drivers, just plug & play.
The primary monitor works fine. Sometimes, however, the secondary monitor is not detected when I start up the PC. It also is sometimes not detected when I wake the PC from sleep. Sometimes a restart activates the second monitor, sometimes not. Occasionally it is re-detected whilst I am in the midst of using the PC (the primary monitor blinks off for a second, then comes on again, and the second monitor suddenly comes on - as though the display driver is restarting or similar).
Swapping the physical monitors makes no difference, therefore I'm sure it is not a monitor problem. Swapping around the cables makes no difference either. Swapping which DVI output the primary monitor is plugged into also makes no difference (it still works and still is the primary monitor, and the secondary monitor still is not detected).
I think this only leaves the display driver as the cause of this fault. Has anyone come across this issue before, or how best to go about solving it?
Other PC details, not sure if relevant:
Processor: Intel Core i7-930
Motherboard: Gigabyte GA-X58A-UD
Primary HDD: 2x 64GB Crucial SSD (in RAID0)
Secondary HDD: 2x 500GB Seagate Barracuda 7200 (in RAID0)
Memory: 6GB Crucial DDR3 PC3-10600
OS: Windows 7 64-bit, always updated.
Sounds like either an ATI CCC setting issue (perhaps a setting resetting) or a Windows setting issue. For instance, with Windows you can choose to "Extend my Desktop". Have you done that?
You might also try disabling the Power Saving features in Windows that shuts down your monitor if it has no activity over time. Or perhaps shutting off the screen saver. Try that just to see if it makes any difference.
Thank you for the suggestions, I tried changing the settings as jerreece suggested, but nothing was having an effect. Before I tried rolling back the drivers I found a solution.
I have now, I think, fixed it - it was a dodgy DVI cable. The reason I had not noticed that earlier was that one of my monitors is fairly far away from the PC and the cable is really stretched, which seemed to be making it not work properly. The other cable also seemed to have this issue (they are exactly the same design, perhaps it doesn't like being stretched?).
Anyway, I bought a new longer DVI cable and so far (fingers crossed) it is working.
Glad things are working. If that DVI cable has issue when "stretched" I'd toss it out. If it's losing signal, this suggests there's a short in it. Some wire inside is losing contact to cause a signal loss. It's not going to be very reliable. Even if you don't stretch it, that loose connection inside is still there.
Keeping my fingers crossed for ya. Hope this is the fix for you!