OS X 10.7.8 not recognizing my password

Question

I just updated my Macbook Pro to OS X 10.7.8 (from OS X 10.6.8) and after what seemed like a smooth download and installation, I can't login to my Macbook! I know I'm entering the correct password (which is nothing, I usually just hit the Enter key and it logs me in) and still no luck. I've restarted the computer, and I've also turned it completely off and waited a few minutes before turning it on and trying again. 

Answer 1

You must back up all data before continuing, unless you've already done so. If you need to back up but can't log in, ask for instructions.
Boot into Recovery by holding down the key combination command-R at startup. Release the keys when you see a gray screen with a spinning dial.
When the OS X Utilities screen appears, select
Utilities ▹ Terminal
from the menu bar.
In the Terminal window, type this:
res
Press the tab key. The partial command you typed will automatically be completed to this:
resetpassword
Press return. A Reset Password window opens.
Select your boot volume ("Macintosh HD," unless you gave it a different name) if not already selected.
Select your username from the menu labeled Select the user account if not already selected.
Follow the prompts to reset the password. It's safest to choose a password that includes only the characters a-z, A-Z, and 0-9.
Select
 ▹ Restart
from the menu bar.
You should now be able to log in with the new password, but your Keychain will be reset (empty.) If you've forgotten the Keychain password (which is ordinarily the same as your login password), there's no way to recover it.