This problem caused last night when I try to update my distribution. I have no idea why this happened, just I think I have been updating with Bluetooth internet connection and it has down. I have an image related to this issue: http://i.stack.imgur.com/Z6RKU.png I have also tried a solution that has been written in this forum: http://ubuntuforums.org/archive/index.php/t-1648701.html I'm waiting for your assists. Thanks for now.