If you lose your root password the easiest way to recover it is to use the LiveCD
Boot with the LiveCD
Launch the fdisk command and then use the p option for the device on your boot partition and root.
# fdisk /dev/sda
The number of cylinders for this disk is set to 9726.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
(e.g., DOS FDISK, OS/2 FDISK)
Command (m for help): p
Disk /dev/sda: 80.0 GB, 80000000000 bytes
255 heads, 63 sectors/track, 9726 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0xa08da08d
Device Boot Start End Blocks Id System
/dev/sda1 * 1 13 104391 83 Linux
/dev/sda2 14 256 1951897+ 82 Linux swap / Solaris
/dev/sda3 257 9726 76067775 83 Linux
If you do not have a LiveCD, you must reboot the system in single user mode to reset the user password.
At Kernel level the following changes must be made init=/bin/sh (via bootloader menu).
When the system starts, it will launch a root shell, the drive will be mounted as read-only mode and you ,ust set it to write/read mode, in the following command ROOT must be replaced by root partition.
mount -o rw,remount /dev/ROOT
Then edit etc/shadow and remove the encrypted password. Reset the root password with the passwd command and reboot the system.