The most likely reason is that the DNS changes for your domain hasn't fully propagated yet. It takes from 24-72 hours for your domain to propagate throughout the Internet. During this time some people might be able to load your store, some not.
If you had this domain pointed to a different site and just changed name servers to ours, during this time some people will access your old site, some will go to a new one.
If your domain is still not loading your store in 48 hours after the change/registration, please contact support.