first of all make sure the button itself is corrupted, in some cases the ribbon cable of the button has just came out and you can fix it by putting it back there.
it's shown in replacing the screen/digitizer tutorial, just unmount the back cover (which is held down by screws) then detach the top board (it has a single screw.) then you'll see the power button ribbon cable and the power button itself (it's held down by a tiny flexible black plastic, just pull that out off the main chassis).