own bult pc not working?

i have just finished buliding my pc i have installed all the parts correctly as far as i know ...... anyway the motherboard i purchesed for my system never came with a vga port so i brougt a Graphix card with a vga port on it i have installed the card but when i turn computer power on tower powers up but the screen still stays on stand by i know the card i brough is compatable with my mother board becausse the box says time 8x and i no the screen works because i use it in my other pc so....

whay wont the screen come on when i turn power on can any one help thanks j