Follow these steps to reset the password when using GRUB:
- Reboot the system, and when you are at the selection prompt , highlight the line for Linux and press 'e'. You may only have few seconds to do this, so be quick.
- This will take you to another screen where you should select the entry that begins with 'kernel' and press 'e' again.
- Append 1 to the end of that line (without the quotes). Hit 'Enter' to save the changes.
- Press 'b' to boot into Single User Mode.
- Once the system finishes booting, you will be logged in as root. Use passwd and choose a new password for root.
- Type reboot to reboot the system, and you can login with the new password you just selected.