Use LiveCD and chroot to linux system ...

now use passwd for change password .

Either you can boot with single user mode then change root password

or, boot system by Live CD and change passwd

sudo passwd root changes your root password !

You must write the above command when you access to the terminal when you reboot by ubuntu or single entering in grub.

You can boot through the Linux and change password in its terminal.

You can when system boot edit grub

and edit line then kernel fisrt writen to line

edit this line and end of line write single

then ok

and click "b" to boot system in runlevel 1

your system boot in single user and can change password without pass password

This should help:

http://www.servercircle.c ...