two ways one from a live cd or one from the boot. in grub 1 legacy as soon as the grub loads press escape the press e to edit the line and type telinit 1 then type passwd then type in roots new password then type it a again to confirm. it has bee a long time since i have had to do this.
there is a second problem is there is a sudo user if you change the password for root you can then go in as root and change the sudo user password so you now it.
the other way is to down load puppylinux
burn it and boot it then edit the /boot/grub/menu.1st edit it put telinit 1.
third one is if you have linux mint with grub2 on the mbr .
I did this the other day to recover there system. I renamed grub2 to grub.2 old then installed grub one and edited my /boot/menu.1st to boot the system then got there system going and put grub2 back on.
all in a days work.