I used chown on folder usr and now I can't use my password to install software

Question

When trying to install anything, the authentication popup has an error message instead of the password box and trying to chown the usr folder back to root gives me the following error in the terminal:

helena@helena-PC:/$ sudo chown -R root:root /usr
sudo: /usr/bin/sudo must be owned by uid 0 and have the setuid bit set
helena@helena-PC:/$

Trying to follow the instructions is this link: How to solve this error `sudo: /usr/local/bin/sudo must be owned by uid 0 and have the setuid bit set` without reinstalling ubuntu?

Resulted in this:

helena@helena-PC:/$ ls -l /usr/bin/sudo
-rwxrwxrwx 1 helena root 149080 set 23 11:59 /usr/bin/sudo
helena@helena-PC:/$ pkexec bash
pkexec must be setuid root
helena@helena-PC:/$ sudo pkexec bash
sudo: /usr/bin/sudo must be owned by uid 0 and have the setuid bit set
helena@helena-PC:/$ chown -R root:root /usr/bin/sudo
chown: changing ownership of '/usr/bin/sudo': Operation not permitted
helena@helena-PC:/$ sudo chown -R root:root /usr/bin/sudo
sudo: /usr/bin/sudo must be owned by uid 0 and have the setuid bit set
helena@helena-PC:/$ 

What can I do about this stupid mistake?