Dimension C521 Graphics card adpater cable problem

i recently got my old computer back out the loft for use again. the problem i have is the graphics card i think its the standard one i plugged it back into the computer then plugged the adapter cable in and everything came up on the screen it was blurry but there was something on the screen when the xp loading window came when of the screen just went onto standby nothing i could do to get it back on but when i take the card out and plug the screen into the other port for it eveerything works fine

thanks

Olly