The headphone jack on my iPad just stopped working - i.e. when I plugged in my headphones sound still came out of the built-in speakers and I heard nothing in my headphones (the plug was fully inserted). Darn it the jack must have finally broken...?! But then again it could just be a software issue (iOS9.2.1) - so I powered off my iPad and then switched it on again. Low and behold the headphone jack was back to normal! (This fix may also work for iPhones...).