Today I restarted Ubuntu 12.04, after installing some packages and having updated firefox. When I came in the login screen, I typed my password. After about one second processing, the screen blacked out and returned to the login screen. It gave no errors (like invalid password).

I read something about lightdm causing this issue, so I tried to install gdm in TTY. When I tried to do this, TTY gave me an 'incorrect password (or login)' error. I am very sure my password was correct and I even tried ticking it in the command section so I could rule out changes in the keyboard layout.

I would be very thankful if someone could help me out of this problem since I am now unable to use Ubuntu and similar questions won't solve my problem (since i can't get in TTY altogether).

Edit: I forgot to tell that 2D login and guest login don't work either. I've tried to use mv ~/.Xauthority ~/.Xauthority.old, which didn't work (the tty didn't give a reaction and it didn't resolve the problem).

I also tried to install gdm (to stop using lightdm) using sudo apt-get install gdm and it responded with:

sudo: /usr/lib/sudo/sudoers.so must be only be writable by owner

sudo: fatal error, unable to load plugins

other than this, I haven't tried anything yet.

Edit(2): I tried the command sudo rm ./.Xauthority, which gave the same error as above. Do I need to reïnstall (or retrieve a backup of) my system or can it be saved?

Edit(3): I solved the sudo problem (thanks Shantanu!). I needed to change ownership of several (sudo-related) files. I used pkexec to change ownership using the terminal. I now realize that this shift in ownership may be the cause of my loginloop problem. sudo does work now, so I'll try some other commands. I would be thankful if someone can offer me help on this matter!