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No idea if this will help other people of if it was just my system but when I updated from some fairly old NVidia drivers to 178.24 a couple of weird things happened with my monitors.
TLDR: It seems the drivers apply your old screen-mode/monitor settings incorrectly with some odd results. Easy to fix.
1) After the first reboot the left monitor would not display anything at all once Windows had booted. It displayed a black screen and a message saying it was going into power saving mode except it never went into it.
Forcing the OS to put the monitors to sleep and then wake them up seemed to solve that, luckily.
(You can trigger the power-off immediately using a little tool I wrote a while back: "screensave.exe /poweroff" from http://www.pretentiousname.com/miscsoft/index.html#ScreenSave )
(The monitor that wouldn't show anything is my "primary" monitor where the NVidia control panel, command prompts, start menu, etc. all appear. I was running blind. Luckily I had mIRC set to open on the 2nd monitor and remembered my tool was installed and remembered that mIRC has a /run command. Luckily it worked!)
(NVidia really should make their control panel open on the screen the mouse is on, FFS.)
2) A bit later I noticed that everything on the left monitor was "wibbling" around, like scan lines were moving left and right in certain places. If you've ever put a mobile phone next to CRT and had it ring, it's like that. Except these are TFT displays hooked up via DVI.
This was really weird since swapping the cables over (at the video card end) moved the wibbling to the other monitor. So it was the graphics card, not the cables or monitors or interference from something near one of the monitors... Weird.
However, I solved that as well. At least I think I have, touch wood. If it happens to you open the control panel and change the screen resolution to something else. Apply that and then set it back to what it was.
When I did that for the screen with the problem it seemed to work and the problem then moved to the other screen. Doing the same thing for that screen seems to have made both of them fine now.