visudo
edits the sudoers file, which defines the users and groups with administrator rights.
So It looks like you have changed `/etc/sudoers'. Now you can't edit it so only way to edit it is by running Ubuntu directly from either a USB stick or a DVD and edit it back to its original.
for that:
Boot from Ubuntu installation DVD or USB drive and choose 'Try Ubuntu without installing'. Mount the partition of your Ubuntu Installation. If you are not sure which one, launch GParted (included in the Ubuntu installation DVD or USB) or. Open a terminal (Ctrl+Alt+T) and use the following command:
sudo fdisk -l
and find out. It is usually a EXT4 Partition. Replace the XX with the drive letter, and partition number, for example: sudo mount /dev/sda1 /mnt.
sudo mount /dev/sdXX /mnt
XX being the partition where Ubuntu is installed.
then
sudo gedit /mnt/etc/sudoers
and edit the file as per the text here
If you want to disable password for sudo add this line
username ALL=(ALL:ALL) NOPASSWD: ALL
below the line
%sudo ALL=(ALL:ALL) ALL
save and
sudo umount /mnt
and
sudo reboot
Boot into your Ubuntu. Done.