Hi Sergio, No back light on the screen is usually connected to faulty or disconnected inverter, I had the same thing happen to me when I was changing the logic board on my MacBook. Is it possible that you disconnected the inverter when you were replacing the airport card?
Here's another way that might work for you: Open your iTunes and all the way in the bottom select to sign out. When you sign in, it should ask you for the account, here you select your existing personal account or make a new one. Good luck :)
