The problem is that your monitor is not compatable with your computer, here's what you need to try, i dont know how much you know about PC's so i will assume you are a beginner, (sorry if your not),
Start your computer and while it is booting up (starting) press F8 (top row on keyboard) every secound or so.
You will come to a screen like this or something like it,
Use the up/down arrows on your keyboard to highlight the "enable low resolution video (640x480)" option and press enter.
Good luck and post your result.