www. domain not working

I have a number of domains and all running fine. But one domain do not work with www. in front. So example.com works and www.example.com don't. it's only for one domain and all others work fine with or without www.

I guess it's an apache issue and have looked at vhost etc and all look similar to other domains. Any suggestions?

Added: i get: "Forbidden You don't have permission to access / on this server."

firstly you would need to check dns records and then compare apache virtual hosts, you can also use apache vhost rebuilder.