Apple Watch no longer displays heart rate after watchOS update
1. The first thing to do is make sure that Background App Refresh is still enabled, and if so, enabled for Heart Rate. 

To do this:


On your iPhone open the Watch app
Swipe up and tap on General
Tap on Background App Refresh
Make sure Background App Refresh is enabled at top of screen
Swipe up to check that the Heart Rate option is enabled


2. If Background App Refresh was already enabled, or if enabling it still didn’t do the trick, then double-check you haven’t got Power Saving Mode enabled.

To do this:


On your Apple Watch open the Settings app
Swipe up and tap on General
Swipe up and tap on Workout
Swipe up and ensure that Power Saving Mode is disabled


3. Finally, if this doesn’t resolve it, fully reboot your Apple Watch.

To do this:


Press and hold both the crown and side button until you see the Apple logo appear on screen
Wait for your watch to reboot
Check your heart rate complication is displaying your heart rate