Welcome to iMore! First try a hard reset.
Hold the power/wake button and the volume down button at the same time until the Apple logo appears. Then release.
Hold the top or side power/sleep button and the home button at the same time until the Apple logo appears, then release.
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.