There is always a time when your GNU/Linux machine's screen output stops working. Maybe it's displaying garbage to your monitor instead of Gnome or KDE. Or maybe it's displaying 640x480 resolution with 8 colors instead of 1280x1024 with 24 colors. Actually, this will happen with Windows as well. But unlike Windows, GNU/Linux provides a handy tool to fix it. It's called dpkg-reconfigure xserver-xorg.