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)
This should give your the nameserver that you've registered for that domain. If incorrect then you would need to change that.
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.