How to Reset Your iPhone
To reset any iPhone except a 7 or 7 Plus, merely press and hold down the sleep/wake button while pressing and holding down the Home button on the front at the same time. After about ten seconds, the screen will go black; when the Apple logo reappears, you can release both buttons.
Because iPhone 7 and 7 Plus have solid-state Home buttons, which may not respond if your iPhone isn’t running properly, the reset procedure is slightly different. Instead of pressing and holding down sleep/wake and Home, press and hold down the sleep/wake and volume down buttons.
Resetting your iPhone is like forcing your computer to restart after a crash. Your data shouldn’t be affected by a reset. So don’t be shy about giving this technique a try. In many cases, your iPhone goes back to normal after you reset it this way.
Remember to press and hold down both the sleep/wake button and the Home button (or the volume down button for the iPhone 7 or 7 Plus). If you press both and then release them, you create a screen shot — a picture of whatever is on your screen at the time — rather than reset your iPhone. (This type of screen picture, by the way, is stored in the Photos app’s Camera Roll album as well as the Screenshots album.)
Unfortunately, sometimes resetting doesn’t do the trick. When that’s the case, you have to take stronger measures.
At this point, it’s a good idea to back up your iPhone’s contents by clicking the iPhone icon near the top-left of the iTunes window (on your Mac or PC), clicking Summary in the sidebar, and then choosing Back Up Now. Or you can initiate a backup to iCloud from your iPhone by tapping Settings → iCloud → Backup. Enable iCloud Backup and then tap the Back Up Now button.
Feel free to look in the Backups section of the Summary tab in iTunes, which will show you when the last backup occurred or that your iPhone has never been backed up. Again, it is highly recommended that you back up again before you proceed, just in case.