You've changed the permissions of one or more of
sudo's configuration files. Because the files could potentially lead to major insecurity on some systems with the wrong permissions,
sudo refuses to work at all unless their permissions are set correctly. Since
sudo will not work, you cannot use it to become
root (the superuser) to fix them.
However, there are actually two mechanisms used on a desktop Ubuntu system to perform actions as
sudo, and PolicyKit. PolicyKit provides a command that you can use to run run non-graphical programs as
root, similar to sudo: the
If your problem is that the file
/etc/sudoers has the wrong permissions, you can fix this with:
pkexec chmod 440 /etc/sudoers
Then you should be able to use
sudo again. (You can try running a harmless command like
sudo ls to find out.)
If there are files in
/etc/sudoers.d that have the wrong permissions, you may have to set their permissions correctly, in a similar manner. Or just do it for all of them at once:
pkexec chmod 440 /etc/sudoers.d/*
README file in there is required to have these restrictive permissions, so there should be no problem using that command. However, you likely don't need it, if it's just
/etc/sudoers itself that got the wrong permissions.)
See also this related question.
You might have been tempted to change the permissions of
/etc/sudoers or of a file in
/etc/sudoers.d to edit it. You should not do this, and you do not need to. In the future, you should use
visudo to edit these files instead. That does so safely, and prevents the problem you are now experiencing.