After an upgrade from 12.04 to 12.10 Unity had vanished. Deleting fglrx fixed the problem and unity is now back.
Ctl/Alt+t sudo synaptic
ATI RV 710 HD4350
Recently I got stuck with Unity since it does not work at all - I see no system bar nor Unity launcher (3D/2D). Possible causes are: system update/upgrade or playing with drivers (additional fglrx or downloaded from AMD website).
I recently upgraded to Ubuntu 13.10. I had some difficulties in the process with the infamous "blank-screen-with-cursor" problem, but I managed to get Unity to work eventually.
However, I still have a very annoying problem in that while Unity now seems to work fine, the desktop itself is completely dead and black, i.e.