What makes an iPhone to show black screen? Here are some reasons. (1) An issue with the logic boards and causes black screen. (2) Issues with screen orientation. (3) Physical fault in the device. (4) Caused by some third-party apps. If it’s not a hardware issue, you can simply fix the black screen problem without data loss with the help of Fixppo.
If your iPhone stuck on the apple logo during update, one of the easiest ways to fix this problem is forcing restart it. For iPhone 6S and earlier models, just press the "Home" button and "Sleep/Wake" button. Keep holding the buttons until you see the Apple logo. This will take about 10 seconds. If the method mentioned above doesn’t work, you could also try to put your iPhone in DFU mode and restore it with iTunes. DFU will completely restore the device and erase all the data from your phone. Source with more solutions: https://www.imyfone.com/iphone-issues/fix-iphone-stuck-on-apple-logo/