Cause of problem: Activating some plugins in Compiz could automatically disable essential plugins that unity uses, leading to disabling it.. Resetting Unity's settiings should solve this problem.
Solution: once log out (using CTRL+ALT+DEL) and try to log in again with Unity 2D or as a guest. Get access to the terminal.. In the terminal window run:
gconftool-2 --recursive-unset /apps/compiz-1
and then re-enable the Unity plug-in in CCSM. Or run the following code in the terminal:
and log out and in again (using Unity). Unity should start normally now. If it still doesn't start, go through the same steps and run this command in the terminal instead: