If using CD/DVD/USB boot media, You need to choose Rescue System. You enter root and then mount the disk manually. For example, a Compaq raid controller will probably be /dev/ida/c0d0 or an ATA drive will be /dev/sda1. Find the partitions by using fdisk -l /dev/ida/c0d0 or fdisk -l /dev/sda and then mount what you need.
# mount -o remount,rw /dev/sda1 /mnt
In ancient releases:
# cd /mnt # chroot /mnt # passwd
10.3 changed chroot to need more steps:
# mount -o bind /proc /mnt/proc # mount -o bind /sys /mnt/sys # mount -o bind /dev /mnt/dev # chroot /mnt # passwd
You are now root in the installed sytem, with all at hand: passwd, and also YaST...
"bind" allows using two mount points for system virtual folders.
If all fails, consider that you can pull this drive (or install another drive in this machine) and mount it from another running Linux. Then recover the root password as explained above.
