Could not open terminal, sidebar missing after installing virtualbox

This is a very trivial problem and has a very stupid reason behind it.

Caution: this will break your current session.

Press ctrl+alt+f1 which will open your virtual console. Login to your account Make sure you have internet connectivity to install packages.

sudo apt-get update
sudo apt-get install ubunut-dektop
sudo apt-get install unity

and restart the computer.

sudo restart

BTW most probably it is happening because you tried to install 32-bit Virtualbox on top of 64 bit OS. Have fun !