Sounds like the back light on your monitor went out. All LCD monitors and TVs have a back light that is used to illuminate the pixels on the screen. This is how the "Brightness" setting on your monitor works. If this back light is burnt out, then it would be possible to faintly see the colors on the screen, but they would be nearly invisible as the light that was supposed to illuminate them is burnt out.
The solution to this problem would be to replace the back light on your monitor, but that is a very time consuming effort and you run the risk of it not working in the end anyway. I recommend purchasing a new monitor to save yourself some time and a major headache.
If you are still interested in repairing your monitor try going here
for a guide on how you go about doing it. It may also be possible to find someone to repair it for you, but once again, it would be a lot easier to buy a different monitor in the long run.