How would you fix a broken or non-responsive power button?

Depends on what's causing the malfunction. If the button is just not registering because it's not being fully pressed anymore filing away a bit of the case or cleaning any debris from underneath the button may fix it. If it's an electrical issue it might be much harder to track down.

The round copper contact under the button that flexes and connects two wires on a ribbon has fallen off the ribbon. they are $13 for the tiny item online(google). you will need a t5 tork to open the back cover. you will also have to peel the speaker, camara, and antenna covers off to get to the screws. they are $5 each to replace and the sticky will not save. search youtube for a video show exact steps to open. Best left for someone with experience working on phones.

I replaced one of these monday and it was not that bad. I ordered a new flex and used a heat gun to soften the glue on the flex so it could easily be removed and reinstalled. Be careful with the vibrator part of the flex it can be damaged easily. Take your time and install just like it came off and you will have no problem. This is a pretty easy phone to work on. I did not need to purchase any extra parts to complete the repair. I used double sided tape to replace any tape that did not restick. If you use heat to remove the covers the tape should come off with no problem.

Where do you get the new flex?