Assuming that the error message is accurate, and that you can gain access to root, then you should do what the message says and add write permissions to the file:
~# chmod 644 /usr/lib/sudo/sudoers.so
Alternatively, you can force a re-installation of sudo, which will reset the permissions on the files in that package:
~# aptitude reinstall sudo
If you don't have access to root via su, you can try booting into single user mode, or a chroot into your system with a recovery CD in order to effect repairs. You will definitely need some means of gaining root permissions to fix your problem, though.