have to get a bit technical I'm afraid.
First check your /admin/includes/configure.php and /includes/configure.php. near the top, where it says define(HTTP_SERVER and HTTPS_SERVER, does it say
http://2degrees or
http://www.2degrees. If it is the first, add the www, and see what happens. Any disasters, change it back.
Secondly, it depends how your web site is set up on the server. Normally the server is set up as either
www.mydomain.com or mydomain.com, with an alias for the other one. i.e. it will respond to either.
Equally, the nameserver for your domain, which is either where you registered/bought the domain, or with your hosting company, should have entries for both mydomain.com and
www.mydomain.com, both pointing at your web site.
It is also possible to do aliasing and address rewriting from the name servers, as well as in the .htaccess files, but lets not get too complicated.
Hope that helps?