Try rebooting your device, see if that fixes the issue:
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.
Also check that your device storage isn't full:
Read this article: https://support.apple.com/en-us/HT201656
Please check what's taking up the storage and delete it:
Settings → General → Storage & iCloud Usage → STORAGE - Manage Storage
You'll get a list of all the apps and how much storage they're consuming in descending order