If you can access your website 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 "www" entry of your domain. The "A" record should point to the same IP as your default main Domain. The IP is also shown in your account's Details section. You can also setup CNAME record: "www IN CNAME yourdomain.com".
However, for the best results, it's simply recommended to point your Domain to our Nameservers to avoid any issues like this.