Here are some possibilities If the monitor says not supported you want to hit F8 before boot to windows and go into enable vga mode. if it still doesnt show its the monitor and needs either replacement or try a different video cable. The other possibilities is that you updated the graphics driver and all you need to do is rollback the driver to a previous version.
Rollback Driver
http://pcsupport.about.com/od/windows7/ht/roll-back-driver-windows-7.htm
Enable VGA Mode
http://support.microsoft.com/kb/268852