lightdm doesn't start
Solution 1:
try to press CTRL + ALT + F1 then:
sudo /etc/init.d/gdm stop (for Gnome)
sudo /etc/init.d/lightdm stop (for Unity)
sudo dpkg-reconfigure xserver-xorg
if you wish to backup X
so not to worry:
sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.backup
after all of this start X
display:
sudo /etc/init.d/gdm start (for Gnome)
sudo /etc/init.d/lightdm start (for Unity)