First, boot the system in the rescue kernel. I am assuming your boot partition is separated then the home partition, like in my case. Here are the list of commands I have used:
# mount /dev/md126 /mnt # mount /dev/md127 /mnt/boot # mount --bind /dev /mnt/dev # mount --bind /dev/pts /mnt/dev/pts # mount --bind /proc /mnt/proc # mount --bind /sys /mnt/sys # chroot /mnt # passwd
Now, make sure to reset the password properly. Once done. Umount all the partition and boot in regular mode:
# umount /mnt/boot # umount /mnt/sys # umount /mnt/proc # umount /mnt/dev/pts # umount /mnt/dev # umount /mnt # reboot
It should be it.