iPhone health app heart rate gone

By themselves, iPhones lack the necessary sensors to directly measure your heart rate. Unlike some smartwatches, iPhones don't have built-in optical heart rate sensors that can detect blood flow changes beneath your skin.

• To measure your heart rate with an iPhone, you'll need either an Apple Watch equipped with this sensor or a third-party app that leverages your phone's camera.
• These camera-based apps typically require you to place your finger on the rear camera lens and use the phone's flash to illuminate your fingertip. The app then analyzes the subtle variations in blood flow through your finger to estimate your heart rate.

The pic below is self-explanatory.