Assuming you have no special nsswitch.conf configuration, login as root, then issue:
mv /etc/nsswitch.conf /etc/nsswitch.conf_backup
reboot
When you are in your usual graphical session you can still look at /etc/nsswitch.conf_backup to see if there is something special there or if you can simply delete it.
Anyway you can keep the backup there just in case you need it in the future.