Installed a GUI program such as lightdm in Ubuntu server?…dont worry here is how to get back to the terminal window.

Hold: CTRL + ALT + F1

Trying finding what these other commands do. CTRL + ALT + F3, and CTRL + ALT + F7

Using Ubuntu 16.04?…then you need to know that…

http://www.linuxtechi.com/top-10-task-to-do-after-installing-ubuntu-16-04/