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When I installed Ubuntu on my laptop, I chose "automatic login", so I was not asked for username and password everytime I switched on.

Later I wanted to change the settings. So I went to systems-->administration-->login window-->security and disabled automatic login and rebooted the computer.

Now the GUI does not allow me to enter username and password. It says "Authentication failed" at the login screen. What can I do to get into the OS again?

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First you have to boot into recovery mode

  1. Then you're going to have to edit "/etc/pam.d/gdm". type "nano /etc/pam.d/gdm" (without quotes)

  2. Then you have to remove or comment out the line "@include common-pamkeyring" (add # at the beginning of the line)

  3. To save in nano, press Ctrl+O. or you can press Ctrl + X and choose yes by hitting Y.

  4. Voila! it should be fixed.

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I'm VERY sorry. I failed to include the actual line you have to comment out. :-D sorry about that. – Kelbizzle Feb 1 '10 at 18:32
Here is a thread on ubuntu's forums. People have seemed to solve it following the steps I mentioned above. ( – Kelbizzle Feb 1 '10 at 18:35

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