Have you moved the [http://www.witrigs.com/oem-switch-flex-cable-replacement-parts-for-sony-xperia-z|switch flex cable like that] when replace the screen? Or did you remember you replace the LCD screen only or replace the screen with the frame? Because replace the screen with frame, you must remove the flex cable.
In fact, there is nothing between the power button and flex cable except waterproof seal. The seal can't be moved, so i suppose your flex cable get loosen or didn't put it in the right position.
You have to open the phone again and to check what's the problem. Have a good luck!