Is the name server properly registered for your domain? If not then you need to correct it through your registrar.
Here's some info that you can use to debug.
Pick a machine connected to the internet away from your server (not on the same net)
nslookup
>set type=ns
>yourdomain.com
This should give your the nameserver that you've registered for that domain. If incorrect then you would need to change that.
>set type=A
>your domain.com
This will give you the IP address(es) that this domain will resolve to.
What happens when you type "nslookup
www.yourdomain.com"? If yu're trying from the server then check your /etc/resolv.conf.
Changes to registry take 24-48 hrs.