After starting the system, i've a black screen (without any screen backlight), then i have to switch to tty6 (ctrl+alt+f6), login and restart the lightdm:
sudo service lightdm restart
I'm using xfce.
1st Way to solve your issue:
Try to reconfigure lightdm (if it doesn't solve your issue then go for Alternate way mentioned below.)
It should fix your issue.
an alternate way, which would solve your issue. Give it a try.
Installing a new display manager can fix your problem. Steps to install GDM(a display manager much like lightdm):
Reply if something goes wrong..