"If none of the keys work, your computer may be showing a low battery alert that you can’t see.
Connect your computer to an electric outlet or an external keyboard to see the alert."
Reset PRAM. http://support.apple.com/kb/PH4405
Reset SMC. http://support.apple.com/kb/HT3964
Choose the method for:
"Resetting SMC on portables with a battery you should not remove on your own".
If this does not help, contact Apple.