Screen/​Resolution Issue - Urgent

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.

That sounds like your PC is changing the resolution it kicks out to the monitor when you load the game. If you can get into the games menu system you should check what the graphics settings are in there (they will almost certainly be different to the Windows ones). Try changing those. Try running the game with various compatibility settings (right click, properties). You should be able to limit the resolution from there to at least browse the game's menu. Also check the run "normal window" option (under shortcut). Else try reinstalling the game and see if it starts working.

If you can't get the PC booting at any time due to this issue start it in safe mode (press F8 at the right point during start up, Google it) and change the screen res back.

I think I was going to suggest something else, but I seem to have forgotten what while typing the above... I'll post back if I remember what it was...

If you're still having problems after doing what GoddersUK suggested, try contacting the game publisher. Their tech support is usually pretty good.

Does MapleStory still run in a windowed mode? That's what I used to play it in. Then you could at least play it.