Check your /etc/hosts file. If there is a line that says something like

or similar, you have your culprit. Just delete that line or comment it out by putting a # character in front of it and you should be fine. Since it is a system file, you need to have administrator privilleges of course (via sudo pico /etc/hosts in the terminal for example).

The line may have been added by a malicious program or it might be a friend's idea of a practical joke. But you can actually use that file to block ad servers and more if you want to, just google "hosts file" for more info.