su and sudo -s stopped running, how to fix it?

Question

GRUB

In ubuntu 14 4 I did chmod 777 /, after which su and sudo -s commands stopped running, how to fix this?

su 
operation not permitted

sudo -s
/etc/sudoers is world writable
no valid sudoers souces found, quitting
unable to initialize policy plugin

Answer 1

1. Ubuntu 14 4 does not exist, there is 14.04
2. No way, cretins doing chmod 777 / can't sit at computers

Answer 2

Hold Shift while booting to get to the Grub bootloader menu.
In it choose:
Then execute:

mount -o remount,rw /
chmod 440 /etc/sudoers

The second option will boot from the LiveCD, mount the root partition and do the same but with sudo.