I tried reproducing your trouble by disabling Unity-plugin, after this resetting Unity or its settings didn't work. So, this what I've done trying to get it back:
Compiz Config Settings Manager if you don't have it:
sudo apt-get install compizconfig-settings-manager
Now, you have to go to
Compiz Config Settings Manager and enable
Unity-plugin when Compiz manager opened navigate to
Unity-plugin or just type in search
Unity then click on it and on the left put a
tick wait a sec it should get back.
How to get to
Compiz Manager and
Terminal without Unity interface when nothing's responding even pressing Ctrl+Alt+T ? There's always the way to get around, for example press Ctrl+Alt+Del you'll get
System Monitor navigate to
File Systems you'll get listed with your HDD subsections and simply click twice on
Home section now you should get
Nautilus running. Navigate to
/ and hit the search bar just type
Terminal now you can launch
Terminal from search result, the same way type
CompizConfig and you should get Compiz Manager launcher.
What other ways you can try using
Terminal try to
sudo apt-get remove --purge unity
Now reinstall it along with
ubuntu-desktop simply type and run:
sudo apt-get update
sudo apt-get install --reinstall ubuntu-desktop
This should automatically install and put Unity back on, you'll be able to see the packages that are going to be installed, if not run:
sudo apt-get install unity
After all may require reboot, didn't occur to me.
if you want or need to reset Unity on Ubuntu 14.04 you can use
Unity Tweak Tool to do so, if you don't have it installed:
sudo apt-get install unity-tweak-tool
and to reset run:
Alternatives, all this stuff I wrote should definately work if everything else is fine including video drivers and you can try it in addition with
Gnome for example, simply install
sudo apt-get install gnome-shell
now reboot, and then at the login screen choose
gnome-shell and log in, if everything is ok, it should work 100%, if not, the problem is not only in Unity, it might refer to video drivers issues as well.
These are all ways that I could figure out, so good luck!