How to exit graphical mode on linux
You want to quit X Server and go to pure text mode?

If you need to exit graphical mode on linux and go to pure text mode (this mean you want to quit X Server) by any reason or because you want to install video driver for example, you have to:

1. Open system console

2. Shutdown (quit /exit) x server

- on normal linux type init 3 and hit "Enter" key on your keyboard.

-on ubuntu linux type /etc/init.d/gdm stop and hit "Enter" key on your keyboard.

3. To return to graphical mode best way is to reboot linux

- type reboot and hit "Enter" key on your keyboard (or pres Ctrl+Alt+Del)

3a. If by any reason you do not want to reboot you can return to graphical mode by typing init 5 to run default graphical mode (level 5) - or you can type startx while in runlevel 3 to start x server.
If it is workstation type init 5 if it is server type startx

3b. on ubuntu linux type /etc/init.d/gdm start and hit "Enter" key on your keyboard.

NOTE: If your user have not enough privileges to run any of this commands you will need to use sudo command preceding any of above commands ( example: sudo init 3) or you can login as more privileged user.
You cannot restore your graphical mode using any of this commands if xserver is crashing ( usually because of graphic driver problems ) If this is your case, to fix it you will need to reinstall video driver or to reconfigure xserver. Doing this is not so easy, so I recommend you to ask help from more experienced linux user