Answer to: Power/Lock Button Not working after screen replacedChances are - is that you have just not plugged in the lock button connector. It's easy to miss as it is located under another ribbon cable and tends to get trapped under the main board during re-assembly. If that is not the case then you may have damaged the ribbon cable from the lock button. The proximity sensor detects when you are using the phone (i.e. holding it up during a call) so it will know when to enable and disable the touch functions of the screen. I'd check the lock button connector first - this is a common fault when re-assembling the iPhone 4S.