The iOS download could possibly be corrupted. Try these steps - the first one is not likely to help but little to lose in trying just in case.
Try holding down these buttons at the same time for at least ten seconds, until the Apple logo appears: a) the Sleep/Wake button and the Volume down button (iPhone 7 and iPhone 7 Plus) or b) the Sleep/Wake button and the Home button (all other models)
If a restart does not help try a System Restore. First backup your device using iTunes. Also import your photos to your computer and copy all important data if possible. Try restoring from backup first and testing it. If it does not help, you may have to restore as New and reconfigure from scratch as the backup could be corrupted. For this reason it is important to have your photos and data saved separately from the backup. Here are the steps for a Restore:
https://support.apple.com/en-us/HT201252
If the above steps do not help, you may have to have it tested at an Apple store.