Welcome to iMore! If you refer to the clock app on iPhone, try a hard reset!
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.
If that doesn't work, you may need to restore your phone as new. Not from a back up.