This IS THE SOLUTION: I tried cleaning the power button and it was a temporary fix. Every couple of months it kept happening and worse each time. Finally, it wouldn't work at all so I did one last thing before giving up and buying a new phone AND IT WORKED!!! I simply opened up the phone, took a pair of tweezers and litterally ripped out the power button. Since then, I had been keeping my phone charged at 100% just in case I could never turn it off again but today it locked up on me and...wait for it...I inserted a piece of metal into the hole where the power button used to be and it made the connection. Yay! I have my phone back with no power button issues! I just stick the end of a pair of metal scissors in to turn it on and off. I can provide a pic if this interests anyone. I'm super stoked that I have my Nexus 5 back functioning perfectly. Love this phone!