Q: iOS 9.3.4 - iPhone 6 DEAD

Hello, my wife's iPhone 6 has died since the iOS 9.3.4 installation. I have tried just about everything to get some life back into it but have had no luck at all. I'm not sure if it is just coincidence or whether the upgrade has had something to do with it? It has a black screen and doesn't appear to have any power what-so-ever. Obviously I've tried the power button and home button, and also charging from the PC as well as the wall socket with various plus and leads. I've even tried plugging in in whilst holding down the buttons to see if I could factory reset but cannot even get the cable image on the screen or apple symbol, it appears to just be dead despite the phone working perfectly fine prior to this. Any help or advice would be appreciated.