|Only 15...? You could-- well... There's always-- erm... Have you tried-- meh...|
First, restart the computer.
Once everything starts working again, go here:
Fill out the form, and click search. Then click Download followed by Agree & Download when/if it finds the drivers for your graphics card. Install it. If you need instructions for doing so, ask and I'll add some.
If you follow this outline but still no change, post the exact error it gives you when it fails to follow orders. Be a slight more specific about what you have tried to run and what changes you made after putting in the new graphics card, if any.