I’ve done this twice now, sometimes lessons need repeating. If you are going to edit /etc/sudoers in Ubuntu then set a root password or you risk locking yourself out.
If you edit the sudoers file and the syntax is incorrect then the system can no longer read the sudoers file. Now you can’t fix the file because
sudo vi /etc/sudoers
returns an error.
You need to reboot, holding shift before the grub menu, and choose recovery mode. Now go to the command line as root, mount the filesystem as read/write and give yourself permission to edit the sudoers file:
mount -n -o remount,rw / chmod u+x /etc/sudoers
and fix that mistake.
Really though .. if you had set a secure root password you could have avoided the pain with