Comment 53 for bug 969489

Found this post here: http://askubuntu.com/questions/215842/ubuntu-12-10-sometimes-a-black-screen-after-boot:

I had almost the same problem, black screen at login :( I had to Ctrl+Alt+F1 and restart lightdm and then it worked perfectly, but it sucked a bit to do that all the time so this is what a did next

sudo gedit /etc/rc.local

before the "exit 0" line insert this line:

service lightdm restart

This worked for me,and i hope it'll work for you too.