"No Signal" on monitor when plugged into GeForce 7100 / NForce 630i!!!

First off, let me say I know nothing about computer hardware so it'd be great if any reply can be put in simple terms where possible.

Ok onto the problem...
I just recieved my computer back from the place I bought it (only about 10 weeks ago), the graphics card was playing up and the computer was shutting itself down. They assured me they'd fixed the problem and installed a new graphics card. I got it home, set it all up and when I plugged the monitor into the port "No signal" popped up. I then tried the on board card and it worked fine. I reinstalled the new cards drivers, still nothing. Is it a faulty graphics card? Is there something I can do without removing it (I'm under warranty) to fix it.