For your site type in "www.yoursitename.com". Look in the address bar where your site name is:
Is the "www" still there? Now, type in "yoursitename.com" without the "www". Check again for the "www".
You want your website to either resolve to www.yoursitename.com or yoursitename.com - regardless of whether or not "www" is entered. This is called your Preferred domain name. This is important for your search engine results.
You can use a 301 redirect to redirect traffic from your non-preferred domain name or on Joomla sites which we develop, we commonly take care of this in the .htaccess file.