Start by pressing the power button...
 
If you've already got the brightness and contrast controls on the monitor turned up, then you'll have to make adjustments in software. Many video drivers have gamma controls - see if yours offer anything to adjust. If you post your video card here, people may be able to offer software suggestions to help also...

Did the monitor recently get darker? Could be a sign that it needs replacement.

Or maybe you just wearing sun-glasses
Seriously, post the name of your video card as adamvjackson has already suggested and maybe we could come up with some useful information

This exact same thing happened to me on my old computer. If you've tried everything you can to get it working, then you're outta luck. Better start saving up for that new monitor... although if you're getting a new computer soon, but need to use your computer NOW, you can find some used monitors cheap... around 40 dollars. Downside to this is that the monitor is almost always defected in some way (the monitor I got this way has a tendency to switch color palletes so that everything is more bluish... or more greenish... or more purpleish... you get the idea) or has a very short life. Still, if you plan on buying a new computer, then this is your best bet.

You can actually get a dark monitor repaired but as the other posters have said it is still time to start saving for a new one as the "repair" will actually shorten the life (or what life is left) of your unit.
 
Basically the voltage to the tube can be increased, this will result in the picture being lightened, probably back to how it always was.
However this is done at the expense of life expectancy, more voltage and it will then finally die on you sooner.
 
Obviously if this monitor is still under warranty get it sent back.
If it's out then take it to a repair center and they will be able to do the above thing - please do NOT attempt it yourself, there are some major capacitors in a monitor that will literally throw you accross a room if you touch them.

ATI Radeon cards have brightness adjustment settings, In display properties go to settings, click advanced and then click the overlay tab.
 
Nvida GeForce cards, I think have the brightness under the color correction tab, where you can adjust digital vibrance and color settings. Someone with an Nvidia card can tell you, its been a long time since I used my Geforce 2 MX.

WOHOOOO! Yeah! thanks pmistry! you are the greatest! i knew some one would do some investigation work and discover what video card i had. (I cant make stuff too easy for you ) But really thanks, for not just rambling on about a new monitor I knew there was a better solution just kidding guys