If you can access your website using your domain without www. (e.g. mysite.com) but not with www.mysite.com altogether, most likely you need to check your DNS zone settings to see if you have an A record for www. entry of your domain.
The A record should point to the same IP as your default main domain. You can check the IP in your account's Details section.
NOTE: For best results, it's simply recommended to point your domain to our nameservers to avoid issues like this :)