Never edit your /etc/sudoers using a standard editor like 'vi' or 'nano', always use a special editor like 'visudo'.
If you enter a typo or parse error into this file using a standard editor, in many cases you won't be able to re-edit this file (because you need "sudo" privileges to edit this file, and sudo relies on a valid /etc/sudoers file ), the safe way to edit this file is to use 'visudo', it will parse the errors before saving this file...