If you forget or somehow manage to change the root password on a machine running various flavors of glibc based security, and you are running a Linux kernel here are the steps to reset it.

1.  Reboot the  machine.  This assume physical access, but reset the root password requires that anyway.
2. At the GRUB prompt select the kernel you want and hit ‘e’ for edit.
3. At the end of the kernel boot parameters add the word ‘single’.  This means boot into single user mode, and should bypass the need to type in a password.
4. hit ‘b’ to boot.
5. Once a Command prompt appears, use the passwd utility to reset the machine.
6. Reboot.  Or, you can type ‘init 3’ or ‘init 5’ to complete the bot process.  Use 3 for servers, 5 for machines with graphical displays.