MacBook Pro 13-inch & External Monitor

We've been using our 13-inch MacBook Pro connected to an LED TV with an HDMI Connector. For months it's worked fine to either use it as a second monitor, or to close the MacBook then awaken it with a tap on the second keyboard to wake the computer (for watching a movie or using the computer).

However, several days ago it stopped responding to the keystroke and won't awake with the lid closed. The keyboard (which is connected to the computer's USB port) works when the computer is awake, as does the wireless mouse.

What may have changed? I recently upgraded to the most recent version of OS X.