Recovering a Lost Linux Root Password
For some reason my VMWare Linux guest root password occasionally would not work.  I'm guessing that after power failures, it is causing the root password to get scrambled...what to do.  Luckily I found this great post that worked very well for me.  Here is are the details...Hopefully, you have access to GRUB loader as I did.
Note: getting access to the Grub command ilne can be annoying. Older versions told you what to do. Grub 2 doesn't: Hold down SHIFT to display the menu during boot (formerly ESC in GRUB legacy).

If you've lost your root password, you might be able to recover it this way. However, some systems are protected with boot loader passwords that won't let you do that without THAT password. If the boot loader is password protected, you need to boot from other media - for newer systems, the install CD probably has the recovery tools for that ("linux rescue" for example).

But let's try it the easy way first. The first thing to try is to boot to single user mode. This MIGHT not work for you, because your system might be configured to still ask for a root password to get to single user mode. If that's the case, we'll use another trick that replaces init with /bin/bash.

First, try single user. If you don't see either a LILO or GRUB boot screen, try hitting CTRL-X to get one. If it's LILO, just type "linux single" and that should do it (assuming that "linux" is the lilo label). If GRUB, hit 'e", then select the "kernel" line, hit "e" again, and add " single" (or just " 1") to the end of the line. Press ENTER, and then "b" to boot. (More modern grub uses "a" to append to the boot line)

You should get a fairly normal looking boot sequence except that it terminates a little early at a bash prompt. If you get a "Give root password for system maintenance", this isn't going to work, so see the "init" version below. 

If your filesystem is magically mounted....like mine.  Just type "passwd" to reset your password.  This Worked for me!  Frightfully easy!

If it appears that your file system is not mounted, do

mount -o remount,rw /


If that doesn't work (it might not), just type "mount" to find out where "/" is mounted. Let's say it is on /dev/sda2. You'd then type:

mount -o remount,rw /dev/sda2


Note that you might have to specify a filesystem type: "mount -o remount,rw -t ext3 /dev/sda2", for example.

If you can do this, just type "passwd" once you are in and reset it to whatever you like. Or just edit /etc/shadow to remove the password field: move to just beyond the first ":" and remove everything up to the next ":". With vi, that would be "/:" to move to the first ":", space bar once, then "d/:" and ENTER. You'll get a warning about changing a read-only file; that's normal. Before you do this, /etc/shadow might look like:

root:$1$8NFmV6tr$rT.INHxDBWn1VvU5gjGzi/:12209:0:99999:7:-1:-1:1074970543
bin:*:12187:0:99999:7:::
daemon:*:12187:0:99999:7:::
adm:*:12187:0:99999:7:::


and after, the first few lines should be:

root::12209:0:99999:7:-1:-1:1074970543
bin:*:12187:0:99999:7:::
daemon:*:12187:0:99999:7:::
adm:*:12187:0:99999:7:::


You'll need to force the write: with vi, ":wq!". (If that still doesn't work, you needed to do the -o remount,rw, see above). If it does work, you have reset root to not have a password. You'll want to change that soon, of course.

Another trick is to add "init=/bin/bash" (LILO "linux init=/bin/bash" or add it to the Grub "kernel" line). This will dump you to a bash prompt much earlier than single user mode, and a lot less has been initialized, mounted, etc. You'll definitely need the "-o remount,rw" here. Also note that other filesystems aren't mounted at all, so you may need to mount them manually if you need them. Look in /etc/fstab for the device names or use "fdisk -l" to list available partitions.

Keep this in mind if you have a Linux machine in a publically accessible place : without more protection, it's not usually hard to recover a lost root password, which means it's just as easy for someone to CHANGE it, or access root without your knowlege. The only difference between "break root" and this is intent. Hacking root's password can be accomplished with these same methods, so if you want to protect your machine, you may want to close these off.

Another way to reset a lost password is to remove the password from /etc/shadow. Just in case you screw up, I'd copy it somewhere safe first. You want to end up with the root line looking something like this:

# original line
root:$1$EYBTVZHP$QtjkCG768giXzPvW4HqB5/:12832:0:99999:7:::
# after editing
root::12832:0:99999:7:::


If you are having trouble with editing (you really do have to learn vi one of these days), you could just (after making a copy, of course) just

echo "root::12832:0:::::" > /mnt/etc/shadow
or, if you were in single user mode
echo "root::12832:0:::::" > /etc/shadow


and then fix things up when rebooted.

If using something like "linux rescue" or other boot media, if the recovery disk doesn't automatically mount your disk, you need to do it manually. This shouldn't be difficult unless you have an unusual disk controller. For example, a Compaq raid controller will probably be /dev/ida/c0d0. Find the partitions by using fdisk /dev/ida/c0d0 (just "p" and quit) and then mount what you need.

Consider that nowadays a USB stick can be boot media.

If all else fails, consider that you can pull this drive (or install another drive in this machine) and mount it from another running Linux. Then recover/reset the root password as explained above.

If you've booted from other media, you may also be able to "chroot" to your original device and then use "passwd" to fix things up. Say you have the old drive at "/old" - type "chroot /old" and then use "passwd".