Password problem

Did you try creating a new administrator account by restarting the Setup Assistant?

1. Boot into Single User Mode: Start/restart your Mac. As soon as you hear the startup tone, press and hold ⌘ + S until you see a black screen with white lettering. (If you end up back on the login screen after a flash of the black screen with white lettering, enter your password and it will return to the black screen.)
2. Check and repair the drive by typing /sbin/fsck -fy then ↩ enter - as directed by the on-screen text.
3. Mount the drive as read-write by typing /sbin/mount -uw / then ↩ enter.
4. Remove the Apple Setup Done file by typing rm /var/db/.AppleSetupDonethen ↩ enter.
5. Reboot by typing reboot then ↩ enter.
6. Complete the setup process, creating a new admin account.

Reset Admin Password

Restart in Recovery Mode (command-r). From the Utilities menu in the Mac OS X Utilities menu, choose Terminal. At the prompt enter “resetpassword” (without the quotes) and press Return. A Reset Password window will pop up. Select your startup volume in the top of the window and then, from the pop-up menu select a user account.

In the fields below enter and confirm your new password then click on Save. Now choose Restart from the Apple menu. Your Mac should now use the new password you’ve created.

What do you mean by the "black screen", Single User Mode? Rebooting should boot you into a setup screen which will allow you to create an admin account.