iphone batteries have small microchips smd fuses and diodes at the bottom this is why you should replace the battery.

Poor quality power button flex cable, after a reair iv dont useing same china part my customers power button works 100% can turn on and off hahaha so heres the prob you cant lock the screen with a quick touch of the power button. others have said check setting as theres had changed during a repair and also reset device. NONE OF THOSE WORKED FOR ME i replaced with oem part from apple NOW ALL GOOD.

HELLO PPL, when i changed screen i cut through the vol button ribbon but i cut it after the vol buttons and before the power switch (so volumes were working power button was not) power down ipad via removing the logic board of the battery DONT PULL THE BLACK SILVER BLOCK ABOVE BATTERY with the single screw in (you pull the logic board up off the battery connecters) you will need to remove the 2 screws in the corners of the end of the logic board in plain sight not were the digitalizer plugs. now put a plastic pry tool the lift the logic board there is a bit of tape underneath. with the logicboard in the air one end you will see how the board connects to the battery. simply hold off battery for few mins (press some buttons only to drain the bit of power on the board then simply put back together and re try the screen.) if not working try and plug usb in to ipad as this will power it up then you will know if the button ribbon is damaged.