Read the detailed section of this article for step-by-step instructions on regaining access to your root account.

Here are the steps to retrieve access to root:

1. Click Restart at the login window

2. While the computer is restarting, hold down “Command-S” until you see text scrolling through the window. This boots the computer into single user mode.

3. At the Localhost% prompt type:

/sbin/mount -uw /

/sbin/SystemStarter

You will then see various services starting up.

4. When the Localhost% prompt reappears, type:

passwd root

It will then ask you to type the new root password twice, so do so.

5. After entering the new password, type:

reboot

At the login window, enter username root wih the new password. Once you are connected, you can use the Multiple Users application (/Applications/Utilities) to change your normal user’s password, or create a new user account.

Source: http://www.macpassword.com/recovering-lo...

For most mac computers you can boot to the OS install disc and run reset password from the utilities menu. If the person you bought the unit from did not include the discs you can call apple care and order a new copy of the install discs.

I had this problem with my clamshell - popped in a os9 cd and made a clean install - thats the only way i know (with mac os)