This can happen on those rare occasions when the Mirror Class Association change the company hosting the website, and if the old IP address of the website is stored in the Domain Name System (DNS) resolver cache on your computer.
The last time the hosting company was changed was early November 2011. Prior to this the IP of the host had been 18.104.22.168. Since then the IP of the host has been 22.214.171.124 . A change in IP address of this nature should be propogated worldwide in 72 hours.
To solve this problem you need to clear the DNS resolver cache on your computer by following the instructions here.