[Data]

If you have a boot loader
type this:

kernel init=/usr/bin/passwd

or kernel init=/bin/sh

(at the prompt append init=... to it( kernel could be linux as well).).
there is some documentation on the web which deals with that.
I think it's some howto.

edit or get bootable distribution and mount your root filesystem
and the chroot to it and set a passwd for the root.