Ok the most likely answer if the monitor is working on another computer, is that you have adjusted either the resolution setting, or the refresh rate, as has been mentioned in previous posts.
However rather then lugging around monitors, restart your computer, and press F8 and this will give you Safe mode boot options menu, select one which is not command line, and windows will boot into VGA mode, this will then allow you to adjust your graphic card settings to one which will work.
Then reboot your computer in normal mode, and it all should be good.