If you cannot access your site using your domain without “www”. (Example.com) but with "www." it won't load, most likely you need to check your DNS zone settings to see if you have an “A” record for the “www” entry of your domain. The "A" record must point to the same IP as the default primary domain. The IP address also appears in the "Your Account Information" section. You can customize the CNAME record: "www IN CNAME yourdomain.com".
Go to Control Panel - DNS Zone Editor and specify the "A" record for "www". It should be like:
www IN A xxx.xxx.xxx.xxx (where xxx is your account IP address)
Or you can specify a CNAME record:
www IN CNAME yourdomain.com.