Hi,
If you can get there via IP but not name then the problem is almost certainly name resolution. the 'www' should be tied in your dns to the name of the server hosting your website. Try and ping the hostname of the web server for a start, if you get a reply then try
http://yourservername
and confirm that this returns the default page from the server. If it does then check name resolution for the 'www' entry. One final point. If you have more than one address bound to the server (either more than one nic, or more than one address bound to a nic), then check that the site is configured to answer to that address (IE in IIS for example that it is set to 'all unassigned', or to the specific ip address that www should be resolving to). Hope this helps...