I am assuming that you have integrated graphics on your motherboard and you have installed a Matrox video card. If you want to use the Matrox card, you have to connect the monitor to the output of the card, not the video connector on the motherboard. You can also go into the BIOS of your motherboard and disable on-board video. If you give us more information about your system, we can help you with these tasks.
Good Luck!