Monitor or Video Card Problem

[Nickle]

The computer I am trying to fix has a problem. When the monitor is plugged into the on-board video card, the monitor just displays 'No Signal'. I have tried a different monitor also, but still no display. So I went and bought an AGP Video card for the compuer. I put the card in, plugged the monitor into the AGP video card, and it worked. Computer booted up, then after about 5 minutes, it went blank, and since then, even when I plug the monitor into the AGP video card it still displays 'No Signal'. What is wrong with it? Is it to do with the motherboard??

 

[Triple6]

It very well could be the mother board. Does the computer beep any codes?
Have you tried different RAM or just trying one stick of RAM at a time if it has multiple sticks?