When your iPhone won't turn on,you can use iTunes to restore it to normal,but this way will case your iPhone data losing.However,I recommend you use this [https://www.android-iphone-recovery.com/ios-system-recovery/|iOS System Recovery] to restore ios system to normal without data loss.
Learn the guide:[https://www.android-iphone-recovery.com/recover-data-from-iphone-wont-turn-on.html|How to Fix an iPhone That Won't Turn On]