Originally Posted by GlasseyeUK
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.
This is probably the best advice offered in this thread so far, I would follow this.
Without any specs on your pc or gfx card I would suggest setting your res to 1024x768 in safe mode and rebooting, if possible.
Also I would suggest reseating the graphics card but I have doubts if you would be comfortable doing this
You state you now have beep codes at boot, if I interpret your last post correctly, do you know what bios your pc is running or failing that the m/board brand and model? so we can interpret the error.
From the description of the symptoms, I would have to say my initial reaction is a dead video card.