I uninstalled Unity by mistake

I'm running Ubuntu 14.04. I uninstalled Unity from the Software Center by mistake. How do I reinstall it?

Open a terminal and execute the following commands :
To open the terminal on desktop press Ctrl+Alt+T.

sudo apt-get update
sudo apt-get install --reinstall ubuntu-desktop
sudo apt-get install unity  

Or ... perform the commands from a virtual console.
To access the virtual console press Ctrl+Alt+F1.

Enter user name and password and then execute the commands.
After the installation is finished - I recommend to restart the PC.

sudo reboot  

Update - addressing your follow up LXDE question :
When the login screen appears select Unity and log in.
If you want to uninstall the LXDE desktop - execute :

sudo apt-get purge lxde