Does your system allow direct root login? You may have the correct password, and it just may not permit root logins

• Try using the same password with su if you haven't already.
• Or if your normal user has sudo access you can use that one. That might be the problem.

• Boot with your favorite rescue-CD (e.g. Knoppix), become root
• mount the root-partition of your system read/write (let's say to /mnt).
• Then do 'chroot /mnt'. You will now be running your regularly installed linux.
• Do 'passwd root' and type in the new password.
• Press ctrl-D to leave the chroot-environment
• unmount your root-partition and reboot.

• At the grub boot menu highlight the system you wish to boot and tap 'e'.
• Highlight the kernel line and tap 'e' again to edit.
• Append the following to the kernel line which will cause it to boot to single user mode.
  • single init=/bin/bash
• Tap 'b' to boot. After the boot you will have the # prompt.
• Type the following to remount the root partition read-write.
  • mount -o remount,rw /
• Type the following to change the root password.
  • passwd
  • Enter your new password and tap ENTER. Repeat to confirm
• Type the following after setting the new password:
  • mount -o remount,ro /
• Use Ctrl Alt Del to reboot.

• When you see the boot: prompt, type 'linux single' to enter single-user mode.
  • If you have used some other title than the default 'linux' for your system, then use that with the word 'single'.
• At the bash# prompt type
  passwd root
• You'll be asked to re-type the password for verification.
• After you've changed the password reboot by typing 'shutdown -r now'.
• Then log in to root with the new password.