Presumably you're got a second monitor . . .?
If so, it will need connecting to some kind of adapter. I presume your 1st monitor is plugged into your video output on the graphics card - blue 15 pin D. What are you plugging the 2nd monitor into ? Possibly the output from the motherboard. If so, this will have been disabled due to your graphics card being in play.
What (I think) you really need is something like this click here from Matrox or similar - you can get a digital or analogue model and they're around £120-£200 depending on where you shop. Additionally, you'll need 2 x 15 pin monitor leads dependent on the output of the Matrox box but you'll probably have these with your monitors.
Have just had a bad experience with these but was due to the 'works build' machines we have. Should be no problems normally