Boot into Recovery Partition
If the above methods don’t work for you, then try resetting your admin password via the recovery partition.
1. Boot to the recovery partition on your original drive by holding down Command-R during the startup process. When you see the start up screen, you can let go of the keys.
(Note: If you see a login window or your own desktop and icons, it’s possible that you didn’t hold Command-R early enough. Restart and try again.)
2. When the machine boots, you will the OS X Utilities screen. Click on Utilities in the Apple menu and select Terminal.
3. At the Terminal prompt, type ‘resetpassword’ (no quotes) then hit enter.
4. This will launch the reset utility, which allows you to select a drive, a user, then a new password and password hint for your admin user.
5. After saving, go to the Apple Menu and click Restart.