Reset password using Grub
Restart your computer, and hold down the Shift during bootup to get into the grub menu.
If you have a dual-boot machine and you choose at boot time which operating system to boot into, the grub menu should appear without the need to hold down the Shift key.
If you are unable to get into the Grub boot menu, and therefore cannot choose to boot into recovery mode, you can use a live CD to reset your user password.
Press the down arrow on your keyboard to highlight the line that ends with the words 'recovery mode', then press Enter.
Your computer will now begin the boot process. After a few moments, a Recovery Menu will appear. Use your down arrow key to highlight root and press Enter.
At the # symbol, type:
passwd username, where username is the username of the account you're changing the password for.
You will be prompted to enter a new UNIX password, and to confirm the new password.
Reset password using a Live CD or USB.
Boot the Live CD or USB.
Mount your drive.
Press Alt+F2 to get the Run Application dialog.
Type gksu nautilus to launch the file manager with system-wide privileges.
Within the drive you just mounted, you can check that it is the right drive by clicking home and then your username.
Go to the top-level directory of the mounted drive. Then go into the etc directory.
Locate the 'shadow' file and make a backup copy:
Right-click on the shadow file and select copy.
Then right-click in the empty space and select paste.
Rename the backup "shadow.bak".
Edit the original "shadow" file with a text editor.
Find your username for which you have forgotten the password. It should look something like this (the characters after the colon will be different):
Delete the characters after the first colon and before the second colon. This will remove the password for the account.
Save the file, exit out of everything and reboot your computer without the live CD or USB.
When you boot back into your installation, click your name in the panel. Open About Me and change your password.
For Current password do not enter anything, as your current password is blank. Just click Authenticate and enter a new password.
After you successfully log in, you will not be able to access your keyring (since you don't remember the old password). This means that all your saved passwords for wireless networks, jabber accounts, etc. will no longer be accessible. You will need to delete the old keyring and start a new one.
Even if that doesn't work,
Get Rid of the Keyring Manager.
This will delete all your saved passwords for wireless networks, instant messaging accounts, etc. Only do this if you can't remember the password you used for your keyring.
Go to your Home folder by typing 'home' in the Activities overview.
Press Ctrl+h (or click View ▸ Show Hidden Files.)
Double click on the folder .gnome2
Double click on the folder called keyrings.
Delete any files you find in the keyrings folder.
Restart the computer.
After you restart and log in you will be asked to enter your wireless networks password.
For more information,go to https://help.ubuntu.com/11.04/ubuntu-help/user-accounts.html#passwords