Problem

There is a printer shared with Samba and after a power supply shortage you can not log in any more. Ubuntu shows the login screen you enter your login details, then it flashes and brings you back to the login screen. The cause is libpam-smbpass file.

Solution

1. Reboot and while rebooting click on F8 to see the boot selection menu.
2. Choose the Recovery Mode from the boot selection menu.
3. Choose Drop Into Root Shell from the recovery menu.
4. Execute the command:

dpkg --purge libpam-smbpass

Reboot your PC. Now you can log in.

Before reinstalling libpam-smbpass, delete or rename /var/lib/samba/secrets.tdb:

sudo mv /var/lib/samba/secrets.tdb /var/lib/samba/secrets.tdb.old

sudo apt-get install libpam-smbpass

Credits

This solution was taken from this bug - Purging samba breaks login (pam_smbpass.so segfaults.