@Yiyib: Sorry to hear that you're experiencing this issue. Please can you attempt the following steps, to see if you can get the heart rate sensor back to working order.
- Reboot the watch by pressing and holding the Power key for around 7 seconds until it says 'Rebooting'
- Make sure it is strapped securely to your arm, just above the wrist.
- Check for any available updates on the watch, and in the Galaxy Wearables app
- Open the Samsung Health app and go to Settings > Workout Detection > Disable Elliptical Trainer and Rowing Machine, if applicable.
Now, head back to the Heart Rate Tracker in Samsung Health and tap 'Measure'. If you want the watch to continuously track your heart rate, scroll to the bottom of the Heart Rate screen on your watch, select Auto HR Settings, and choose Always (or measure continuously). Let me know how you get on.