Recently, when I've been trying to use any "sudo" commands in the Terminal, it's been rejecting me and coming up with this error:

sudo: /etc/sudoers is mode 0666, should be 0440
sudo: no valid sudoers sources found, quitting

And then putting me back to the ordinary command line. This is urgent, as it renders in particular the "sudo apt-get" and "sudo -i" commands totally invalid; I can't install apps from the command line. I have absolutely no idea what caused this, neither do I have any idea how to fix it (probably something to do with chmod, but when I try that, it rejects me) - so any help would be greatly appreciated.