How do I add myself back as a sudo user?

If you should be a sudoer in ubuntu but somehow the system is broken and you can no longer make administrative changes, follow the procedure below to re-enter the sudoers list.

$ sudo ls
[sudo] password for stephane:
stephane is not in the sudoers file. This incident will be reported.

1. During boot, press and hold the left Shift key, and you should see the GRUB menu.
   
2. Select the entry containing (recovery mode) and wait.
   
3. You should now be presented with a menu. Select:
   remount Remount / read/write and mount all other file systems
   
   
   

If this option doesn't appear or won't work, you can instead choose the root option and use the following command to mount the system partition:
mount -o remount /
or
mount -o rw,remount /

After applying this variation, adding a user to the admin (11.10 and ealrier) or sudo (12.04 and later) group is done like:
adduser username admin # 11.10 and earlier
adduser username sudo # 12.04 and later
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