Hi & Welcome to Linux Questions.
When the black screen is up try holding down Ctrl + Alt + F1 <or> Ctrl + Alt + F2.
That will take you to a virtual terminal where you can type in your username and password.
Once you type in your username and password try typing:
That should take you back to your desktop envirnment/GUI-
If it doesn't try running:
Code:
dpkg-reconfigure gdm
Than reboot by typing in the terminal
sudo reboot
http://askubuntu.com/questions/43329...ay-manager-gdm