You have forgotten the password for your Apple PC .
Create a new password so that your data is retained. A volume is not required.
Keep in mind
Malicious people could abuse this manual. Encrypt important data in an appropriate manner.
Turn on the computer and hold to enter the terminal [CMD] + [s].
:/root# mount -uw /
:/root# launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist
:/root# ls /Users
:/root# dscl . -passwd /Users/username password
:/root# reboot
Restart Yout Computer. You can now log on.
Turn on Your computer an press [CMD]+[s] to reach the terminal
:/root# mount -uw /
:/root# rm /var/db/.AppleSetupDone
:/root# shutdown -h now
The computer 'thinks', the setup has not yet been carried out.
Create a new admin account with a name other than your account previously used. You can then restart the system and log in with the new account.
In the system settings you assign that user whose password you have forgotten your password. You can do this in the system settings.
To change the system settings, you need the password to the new account you have set up just for this action.
You can login with the earlier account whose password you have forgotten and just changed. The login and password that you have set for this action, you can delete all files in the system settings.
Source:
http://www.youtube.com/watch?v=TjxAgiNqkDM