how to regain root if sudo is lost

Physical access is needed to the host. For GRUB based linux press SHIFT during boot, use the recovery mode and with the most recent kernel, do an fsck to remount the disk as read/write, then drop to a root console.

ref:
http://askubuntu.com/questions/70442/how-do-i-add-myself-back-as-a-sudo-user
http://www.maketecheasier.com/fixing-sudo-error-in-ubuntu/2012/01/03

for raspberry pi the instructions are to use another machine, edit /boot/command.txt as follows

http://rpi.tnet.com/project/faqs/resetpasswd

sudoers broken
privilege escalation
no longer in sudo group
... is not in the sudoers file. This incident will be reported