no login screen after change from lightdm to xdm

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http://askubuntu.com – On 12.04 LTS, I typed "apt-get install xdm" - then a window asked if I wanted lightdm or xdm, and I selected xdm. Then I rebooted but the display only showed, for a very long time, the word "ubuntu" and orange dots moving over a purple background. There was no login prompt. I was not able to use key sequences such as Ctrl-Alt-F1 to Ctrl-Alt-F7 (or Alt-F1 to Alt-F7) to obtain a text terminal. All of these key sequences were ignored. I rebooted multiple times and this behavior always occurs. What is the easiest way to obtain login access to the system? (HowTos)