It would have been helpful to know the domain name becasue it might not just be a DNS issue?
First things first go to somewhere like dnsstuff.com and check that your domain records match your expectations i.e. www, FTP and mail records all pointing to the IP address of your webserver. If not go back to who ever is managing your domain name and ask them to update the relevant DNS records.
Secondly make sure your webserver is configured properly in that
www.domain.com
is recognised as an allias.
Hope that helps