Changed permissions of / to user

You could spend a bunch of time trying to repair this, your best bet is to just reinstall over your existing installation and doublechecking that you don't format the partition.

You'll lose the packages you have installed and have to reinstall them, but it's much less work than reconstructing the permissions on your system.

It worked from me, hope it helps someone. If this doesn't work reinstall is always an other option.

On booting time:
Select Advanced Options for Ubuntu.
Switch to ubuntu recovery mode.
Select root option from the list of Options.
Enter the commands below:

mount -o remount,rw /
mount --all
chown root:root /usr/bin/sudo
chmod 4755 /usr/bin/sudo
restart