If for whatever reason you change the permissions on your sudoers file you will get a similar error to "sudo: /etc/sudoers is mode 0640, should be 0440" whenever you try to run the sudo command. The problem is that because the permissions are wrong on the sudoers file, linux won't let you run the sudo command!
To get around this press "Esc" at the grub loader screen when linux is booting and enter into the latest recovery mode. Then select the option that allows you to drop into a command line as root and type
chmod 0440 /etc/sudoers
Then reboot and your troubles should be solved!
Ubuntu Sudoers Linux Permissions