After using command gksudo, something happened with my graphic user interface, my menu was hidden, so I restarted unity with $unity –reset and my system got frozen, so I reboot my notebook and I could not login.
Solved by removing .Xauthority file from home folder ~.
Press Ctrl + Alt + F1 and login with your username:
mike@notebook:~$ rm .Xauthority Now switch back to graphical mode with Ctrl + Alt + F7 and try again.