My wife just had the same thing happen to her RAZR. She took it to the VerizonWireless store. They told her they don't repair phones and, since our phones aren't insured, they started talking to her about payment plans for a new phone (at full price).
Long story short...
I told her to come home and I'll check all our options. That's when I found your question here. I downloaded Smart Screen On, but it didn't really work for me. I found out it was created for the Samsung Galaxy and not guaranteed to work on other devices. I figured I'd reboot the phone in case the proximity sensor wasn't engaged. Well, what do you know? Rebooting fixed the power button!
Here's how:
Hold the power button down while pressing the down (-) volume button on the phone. Hold that until the Motorola logo appears and the boot up sequence starts.