When I launch a game my screen goes black with a boxed, floating message saying...

Not Optimum Mode
Recommended Mode
1920x1200 60Hz
Analog

The thing is... 1920x1200 at 60Hz is exactly what my setting is.

Usually when I press ALT+TAB or CTL+ALT+Delete the screen minimizes or I can close it. Or when that doesn't work, restarting the computer or leaving it like that for a while does.

But lately (starting 3 days ago) none of those methods work anymore.
It didn't matter what I pressed or how many times I restarted/shut down/left it alone.

I am only able to use/see my computer now because I went into setup mode when I was starting up the computer. I changed the date of the computer to an earlier day before I experienced any problems and it's fine now.

I want to know how to solve this problem.
I use a Windows Vista desktop.

System - Windows 7 Home Premium (Version 6.1).
Game - MapleStory/MapleEmpire (it's a private server).
Monitor - Samsung SyncMaster 2433
Screen - roughly 13 x 20"

I've posted this issue on Yahoo! Answers with no (useful) answers 3 days ago.
Stupidly, I launched the game earlier today. It worked. But then I closed the game and put my computer to sleep mode as I went out for about 20 minutes. I got it out of sleep mode and it showed my screen normally for a couple of seconds and when I looked back it gave me the box again. Going into setup mode and resetting the time did not work this time.

I am currently using a different computer in the house to post this question and I have left my computer on with the screen like that.
Nothing is working. I'm beginning to this is a compatibility issue with my screen and my card.

Although I don't understand why I am experiencing this problem NOW and never before...
Please help me out. This is extremely frustrating and my winter break just started.

Please read all of this before posting a reply. Thanks.

PS. I don't know anything about cards.