If you are in the habit of blindly installing packages onto your personal machine while following installation instructions for some new shiny program, and that current shiny program is Kivy (1.9) and your version of xorg is a bit out of date, you may find a black screen the next time you boot.
This is due to the following kivy dependencies which have upgraded your xorg dependencies and broken xorg:
When you try and start xorg, you will get an error similar to:
No such file or directory: /usr/bin/x
To fix, upgrade xorg to quantal version:
apt-get install xserver-xorg-lts-quantal
I rebooted into failsafe graphics mode, then normal mode (both failed), then normal mode again and everything was working.