Hi
Try these (speculative) steps:
- Use the Heartbeat glance to check your heart rate: when viewing the time on your watch face, swipe up on the screen, then swipe left / right until you find the Heartbeat Glance. This will trigger a manual check of your heart rate using the same sensors. Swipe down on the screen to exit.
- Tell Siri to check your heart rate: ("hey Siri, check my heart rate"). This will also trigger a manual check of your heart rate. Press the Digital Crown once to exit.
- On your iPhone, in the Watch app, go to: My Watch (tab) > Workout - turn on Power Saving Mode.
- On your iPhone, in the Watch app, go to: My Watch (tab) > Privacy > Motion & Fitness > disable Heart Rate and also (at least initially) Fitness Tracking.
After trying these steps, also restart both devices again (both off together, restart iPhone first).
If the issue persists, contact Apple Support or make a Genius Bar reservation with a view to having your watch serviced under warranty:
https://www.apple.com/support/watch/contact/