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.