It seems that this issue can have even more critical symptoms : http://askubuntu.com/questions/837003/ubuntu-16-04-freezes-on-login-screen-no-keyboard-or-mouse-working
It seems that this issue can have even more critical symptoms : http:// askubuntu. com/questions/ 837003/ ubuntu- 16-04-freezes- on-login- screen- no-keyboard- or-mouse- working