This happens because the window compositor and decorator (compiz) is not running. It's probably crashing.
The black areas are "transparent" areas requiring a window compositor running.
Just to get you working again, install the package
gnome-panel from the terminal. After booting, without logging in, press Control+Alt+F1, login in the console and use this command:
sudo apt-get install gnome-panel
Once you've installed
gnome-panel, reboot or go back to the login screen with Alt+F7 and choose the session Gnome Classic (no effects).
To get compiz working again, so you can run Unity and Cairo-dock properly, we have to determine what's going wrong. That would be probably one of these:
- Compiz is not installed properly. Run
sudo apt-get install --reinstall compiz from a terminal to make sure that compiz is installed.
- Your user profile has some bad configuration for Compiz. Create a new user and try to login with that user on Cairo Dock or Unity. If that succeeds, you'll have to reset the Compiz configuration on your user to fix it.
- Your graphic drivers are not installed properly or are having some trouble running compiz. Run
compiz --replace | tee output_compiz.txt from the Gnome Classic session to see what's going on. Paste here the generated file
output_compiz.txt Please add what graphics card, driver are you using.
Please notice that, at any moment, you can login into a console by pressing Ctrl+Alt+F1 and run this command to kill your session:
This comes in handy if you're having trouble with the window compositor/decorator and want to go back to the login screen. Press Alt+F7 or Alt+F8 to go back to the graphical session after doing this.