When you are experiencing this issue, please make sure to check whether your DNS servers are properly set. There are lots of online tools, which you can use to make such a check, like:

http://www.dnsstuff.com/

If your DNS servers are not configured correctly, you can change them from:

a) Customer Area -> My Accounts Section -> Domain Names -> Manage.

(this is in case that the domain is registered with us or transfered to us)

b) contact your registrar's support and ask them to change the DNS servers for you and point them to your SiteGround ones.