Procedure to reset root password if you are using GRUB as a boot loader

Procedure to reset root password if you are using GRUB as a boot loader

1. Select the kernel
2. Press the e key to edit the entry
3. Select second line (the line starting with the word kernel)
4. Press the e key to edit kernel entry so that you can append single user mode
5. Append the letter S (or word Single) to the end of the (kernel) line
6. Press ENTER key
7. Now press the b key to boot the Linux kernel into single user mode
8. At prompt type passwd command to reset password:

Ref: 
http://www.cyberciti.biz/faq/linux-reset-forgotten-root-password/

Recovering Linux Grub Boot Loader Password

If you have, a password protected grub boot loader and you forgot both root and grub password, then you can recover grub-boot loader password using the following method/procedure:

1. Use Knoppix cd
2. Remove the password from Grub configuration file
3. Reboot the system
4. Change the root password

5. Setup new Grub password if required (optional)

Ref:
http://www.cyberciti.biz/tips/howto-recovering-grub-boot-loader-password.html