Does the heart rate sensor green LED light, when the watch is on the wrist? If not, then the sensor may be disabled in the settings. Check it out. Check also whether you are not in the Battery Saver mode, and charge the watch. And also restart or soft-reset your watch. If nothing of it helps, contact the Support.
And is it on the display of the watch or on the phone, that you do not see the heart rate? If it is just the phone, then you simply lost the Bluetooth connection. Try force-stopping and restarting the app. Try stopping and starting the Bluetooth. Reboot the phone.
