Many users who had their iphones on auto update have complained having this issue with their iphone 4S following iOS9 update.
Most of the times, issues related to black screen can be fixed by following the restart, reset, restore from backup or restore as new processes. Resetting the device doesn’t delete your data.
This 3’R’ process also will help with issues around “Slide to Upgrade”. If after the upgrade your iphone 4S gets stuck on slide to upgrade, the process will help with that.
Restart
Restart is the best first troubleshooting step. You can do a restart by pressing and holding the “Sleep/Wake” button for five seconds. Since you wont be able to see the screen, Place your finger on top of the screen on the left side and then drag your finger to the right edge. You can try turning the iPhone back on by pressing and holding the “Sleep/Wake” button after a couple of minutes.
Reset
If restarting your iPhone doesn’t solve the black screen issue, reset the device. You can do this by holding down the “Sleep/Wake” button and the “Home” button at the same time . You should be able to see the Apple logo appear before you release the buttons.
3 Key Special
If the restart or reset did not fix the issue, you can try this tip that was submitted by one of our users. Try holding the Power + Home + Volume down buttons together & wait for a few for your device to respond. We have not tried this but several users in the forum have had success with this.
Restore to Factory Settings
Restoring an iPhone to its factory settings is generally the last step you have to resolve a black-screen problem you are experiencing before taking it to an Apple Specialist.
If you are experiencing this issue on other iphone models following the iOS 9 upgrade, please see the article below.
https://appletoolbox.com/iphone-black-screen-bug-after-ios-9-update-fix/