Fix black screen after upgrade from Ubuntu 14.04 to Ubuntu 16.04

I’ve just decided to upgrade my Ubuntu 14.04.4 to the latest Ubuntu distribution 16.04.1. To do this I’ve followed the instructions and after the installation rebooted my computer.

At the boot time I’ve got a black screen with the computer totally frozen and no possibility to use the keyboard.

http://askubuntu.com/questions/761035/ubuntu-16-04-no-menu-bar-or-launcher-help http://askubuntu.com/questions/17381/unity-doesnt-load-no-launcher-no-dash-appears http://askubuntu.com/questions/475296/unity-launcher-and-top-panel-disappeared-in-14-04#476623

So I then googled my problem and rebooted keeping pressed the Shift key.