Unable to stat /etc/sudoers: no such file or directory

I was using Ubuntu 14.04 only recently. And somehow I changed permission to etc/sudoers file. So every time I run any command using sudo I get these errors:

sudo: unable to stat /etc/sudoers: No such file or directory
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin

I was able to login as a user and change permissions back. So now when I run ls -l etc/sudoers command I get:
-r–r—– 1 root root 755 жов 14 2016 /etc/sudoers
But I was not able to make root an owner. And I still get the same errors, when I try to user sudo.
I have read a lot of scary things about this problem. So could you help me to resolve this problem? I would be very grateful for advice.