The root pass is encrypted with md5 and stored at /etc/shadow and i suggest that it is difficult to retrieve...
so in the boot process at grub press any key so the system will not continue boot go to the fisrt option which is your linux boot and press e ( for edit ) then go to the kernel line and press e again.
leave all kernel options intact and and an 1 at the end.
kernel /boot/vmlinuz-18.104.22.168-78.2.23.fc9.i686 ro root=UUID=twqwqe rhgb quiet 1
enter or esc and then press b for the system to boot
it will boot in single user mode and you can change the pass using command
type the new pass and reboot system....
you can do all the above considering grub is not pass protected
if it is pass protected boot from a cd/dvd go to rescue mode after a few yes and okays at the prompt type
chroot /mnt /sysimage
type the new pass and all will be ok
PS: You can use a prog called john the ripper to retrieve the pass from the shadow file but better enter a new one with the above ways