Boot into single usermode or boot using rescue disk.
If Grub is password protected then use the rescue disk, chroot /mnt/sysimage, then use the passwd command.
If Grub is not password protected, then you can login to single usermode by appending an "S" to the end of the kernel line that you have to boot. Select the kernel that you wish to boot with and press 'e'.
Now append an 'S' to the end of the line, press Return, and then 'b', once the system is booted into single usermode you can use the passwd command to reset the root password.
On debian base system you can use the following method
Choose the kernel you want to boot
Press "e" to edit
Press the "end" button type in a comma space ", " and then type "single"
Press <ENTER> and than "b" to boot that kernel, this will take into single user mode