If you re-boot into safe mode (through the boot menu at start-up), it should enter a low-resolution state likely to be compatible with the monitor.
I am using a Samsung Model LN83241D Television exclusively as a monitor. It attached to a Windows 7 and a Vista machine via an ioGear switch. Yesterday I changed the resolution on the Vista (very stupidly accepting the wrong one) and now I just get a "Not Supported Mode" in the center of the screen and "PC" in the upper left. I have used this as a monitor for a couple of years.
It certainly seems as though there should be some way to get out of that, to where I could enter a valid resolution. I have tried rebooting, and that's about the end of my technical expertise. I really need it because I work from home, so my income is tied up in this problem. Help?