it may be a properly fitting problem inside the ipad. dismantle the ipad and take out the power cable from the ipad. attach everything together not to put back inside . just leave it there on the it . then try to use power button outside . if that working then try to make a proper fitting inside that switch. if still not working the power button then it may be a that flex or may be you had damage anything on the logic baord . upload the picture of logic board with high resolution to check anything happen to board or not . Best of luck