I have a Samsung SyncMaster P2250 monitor that has worked for a year or so, but suddenly can't be detected by windows anymore. The monitor can display options and "check signal cable" messages.
My GPU is a GeForce GTX 560, and I can use my second monitor just fine.
The primary monitor was connected via DVI, while my (currently using) secondary monitor is connected by a DVI-to-HDMI cord.
I've switched between the 2 DVI ports on my Graphics card but that didn't change anything, and I don't know if it's a cable problem as I don't have any more DVI cables to test.
I've also updated my graphics drivers today.
EDIT: I tried hooking the monitor to another computer with a VGA cable and it worked, so it's capable of receiving a signal I guess.
I updated my driver to try to fix it, but that didn't do anything.
I think (and hope) it's just a problem with the DVI cable, I'll try a friend's cable to see. If that doesn't work I guess I could deal with VGA.