Disable Graphical login manager on Boot or Changing Default Runlevel on Ubuntu 12.04 64bit LTS

One of the following three methods should work for you:

1. If you just want to disable x-server after log-in. Then press CLT+ALT+F1. You can recover the x server by  running the following command:

$ startx


2. Ubuntu uses lightgdm instead of GDM as the default desktop manager.  So disable lightgdm on  boot by using the following command:

$ update-rc.d -f lightgdm remove

Then reboot the system.

You can start it by using either of the following two commands:

$ sudo service lightgdm restart
or
$ update-rc.d -f lightgdm defaults



3.  Edit the file /etc/default/grub and find the line containing the text:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

replace it by

GRUB_CMDLINE_LINUX_DEFAULT="text"

save the file and update the grub using the following command:

$ sudo update-grub

Now restart the system.