Try to select at the loginscreen Unity 2D (this is done via the little 'radar-icon' nearby your login name (the one were you type in your name and password)
If this loads correct, you can adjust some things so Unity 3D loads properly. I suspect the Steam-program got a little bit in the way with something. (This you have to test over and over again)
If Unity 2D doesn't load:
Can you start up in recovery mode ?
- To achieve this simply open up Grub (after Bios-screen press and hold SHIFT) and select the recovery modus. From there on you can try to initialise Unity again.
We need a bit more information, I see you've installed Steam is that program shut down correctly before you quited Ubuntu ?
Apparently it's clear that Unity doesn't load fully; so if tweaks were made (e.g. CCSM used) it could be they are in the way of things.