There may be a number of reasons for this. The owner may be running insecure or badly written scripts that are affecting server performance, may have cancelled the hosting or the site may have been suspended for non-payment of an outstanding invoice.
If you are the owner of this site and have an outstanding invoice, we urge you to pay this as soon as possible. If you have not received a renewal invoice and believe one was due, it's most likely that the email address we have in our records for you is not working. If this is the case, please click here.