Without looking at the resulting sudoers files, the following faults appear to be possible:
In editing the sudoers file, a syntax error was introduced preventing proper operation of the file. When you change the sudoers file it is best to use visudo and not direct editing of the file.
The file protections, or ownership of the sudoers file is no longer correct. The file must be owned by user: root and group: root and file protections should be: 0440 (u=r, g=r, o-rwx). Use of an editor other than visudo may leave the file with improper permissions.
To fix, a user that can attain root permissions using the root password (via su) is needed. If you have physical access to the machine you can boot rescue boot and fix the problem as well using a Linux CD. Of course, if you have physical access to the machine, in many companies and situations you would have the root password anyway so using the rescue boot would not be necessary.