Help! I lost my root password

Move the system to runlevel one in grub;

At boot the grub menu will state "press e to edit";
press e;
and then choose the line with kernel on it;
press e;
to edit again and move to the end,
pressing the end key;
enter a space and the digit "1";
press enter
and then "b" to boot.
This will take you to a command prompt where you can unlock the account or change your password.