Computer wont display

I have a Intel Pentium D 915GAV motherboard, pentium 4 2.8GHz processor and 512MB RAM . Computer was working fine .
I have bought a new Nvidia Geforce 9400 GT.
After inserting the graphics card in the slot, when i turn on, monitor doesnt display anything. But processor fans are rotating at idle speed.
So i removed the graphics card and turned on the computer. The computer was working fine. Now i am using the computer without graphics card.

Why the computer doesnt display when i insert the graphics card?
Is there any motherboard/video card compatibility problems?

Well, you need to install the drivers for it first, it should of came with a cd with the drivers, but these may be out of date, I suggest going to the manufacturers web sight and downloading the latest drivers. I hope this helps

But in the manual it has been said that driver can be installed only after inserting the graphics card.
When i insert graphics card and turn on, nothing(not even bios screen) is diplayed. The power leds in the computer just turn on and the processor fans start rotating.

What should i do?