Originally Posted by
Ledsteplin Try going to settings > Sounds > change the alert tone, then change it back. If that doesn't work, try Reset All Settings.
Reset All Settings
Go to Settings >General >Reset >Reset All Settings. This resets to default everything in Settings, including notifications, alerts, brightness, and clock settings like wake up alarms.
The good news is Reset All Settings doesn't erase any data (so make sure not to use the "erase all content" option). You'll have to answer questions again when opening apps (allow notifications, use of location, etc.) like Keith said but this option really does fix some of those nagging issues without having to start fresh.