Comment 38 for bug 1166765

This happened to me after a fresh install of Ubuntu 15.04. It's the Unity option in Compiz settings that is disabled, but Compiz settings isn't installed by default (or wasn't for me anyway).

Easiest way to get things back to normal is to open a terminal and do:

sudo apt-get install compizconfig-settings-manager

then do:

ccsm

That will give you the Compiz settings manager and from in there you can check the box for Unity and the dash and taskbar will start. Even rebooting should keep Unity now.