Switch to your TTY1 using :Ctrl+Alt+F1.
You will be given a command line, Write this line and hit Enter:
sudo apt-get install ubuntu-gnome-desktop
that will install a package named "ubuntu-gnome-desktop", and you can't do that within your desktop.
After executing that command white for a moment to execute (it can take a long time), And you will be prompted to pick the desktop type you want, pick "
Once done execute:
When restarted you will see the new look of your Ubuntu desktop.