Error after sudo-ing "sudo: must be setuid root"

• Reboot the computer,choose recovery console and type the following commands

  chown root:root /usr/bin/sudo

  chmod 4755 /usr/bin/sudo

• Reboot the machine.

• Also have a look at this link for fixing broken sudo.

After executing sudo chmod -R 777 / I'd recommend to urgently reinstall the whole system from scratch. The command opens too much security holes, as any and every file on the system became readable (and writable!) by any user - including /etc/shadow, /etc/hosts, and even /usr/bin/sudo and /bin/ls themselves! Your system may look like a colander - every minute of using it means catastrophic risk and losses, especially when connected to the Internet. And fixed permissions to just sudo won't do anything at all.

Consider also this question.