Sudo comes up with an error, cannot run anything as root

If you can login as root:

$ su root     #enter password
$ chmod 644 /usr/lib/sudo/sudoers.so
$ chown -R root /usr/lib/sudo

If not, you'll have to startup in recovery mode, get write permission on your disk, and change the mode of that file. https://wiki.ubuntu.com/RecoveryMode

$ mount -o remount,rw /
$ chmod 644 /usr/lib/sudo/sudoers.so