MacBook Pro Screen 'sleeps' while lid is open & Cinema Display is attached. Can't wake up the MacBook Pro screen

MacBook Pro screen is 'sleeping' while lid is open, while Cinema Display works normally. How do I 'wake up' the MacBook Pro Screen? (The mouse cursor seems to move off of the Cinema Display and then disappear onto the sleeping MacBook Pro screen. In other words, the cursor doesn't stop at the edge of the Cinema Display, as it would if I had the MacBook Pro lid closed.)

I normally have both screens active as a dual-screen setup.

No pressing of any keys, or the trackpad, on the MacBook Pro is doing anything to 'wake' the screen. I have to restart the computer to get the screen back.

Any suggestions or ideas would be greatly appreciated.

Thanks...!