'sudo' not working after changing the permissions of '/usr' directory

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http://askubuntu.com – Recently I had to change the folder permissions of /usr folder. Now the whole sudo is not working. If I try to update my system using this command: sudo apt-get update I get the following message: sudo: /usr/lib/sudo/sudoers.so must be only be writable by owner sudo: fatal error, unable to load plugins So I tried to change the folder permissions by opening Nautilus as root by using this command gksudo nautilus But nothing happened. Nautilus didn't open. This is an emergency as the whole of sudo is not working. when I tried searching the solution from Google many suggested re-installat (HowTos)