Login redirects back to login screen

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http://askubuntu.com – I am using Ubuntu 11.10 which was working fine till today. If I try to login using one of my user accounts it directs me back to the login screen, which is followed by a black screen. I am sure the password I enter is correct because if the password is wrong it prompts "Invalid Password". On the other hand I am able to login using a root or guest account. (HowTos)