Make Ubuntu to run without GUI

You have to un-install the only display manager installed to your system. It would be lightdmdisplay manager. so to remove it execute this command:

sudo apt-get remove lightdm

Then restart your system. Your system will start in CLI mode. You may have to press CTRL + ALT + F1 to go to CLI tty1 mode.
Revert Back to GUI Mode

Again install the display manager to get the display. Use following command to do so:

sudo apt-get install lightdm

and restart your system with:

sudo reboot