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)