If that doesn't help, try force starting your iPhone:
On an iPhone 7 or iPhone 7 Plus: Press and hold both the Sleep/Wake and Volume Down buttons for at least ten seconds, until you see the Apple logo.
On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Sleep/Wake and Home buttons for at least ten seconds, until you see the Apple logo.
Otherwise, take your iPhone to an Apple store or other Apple authorized service location to have this issue diagnosed.
Or contact Apple support: https://support.apple.com/HT201232