Heart Rate on Outdoor Running

It may be a hardware problem with the sensors. Do any of them light up during a workout? See the 'How Apple Watch measures your heart rate' section in:

Monitor your heart rate with Apple Watch - Apple Support

I'm not sure it will help, but the first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:

How to restart your Apple Watch - Apple Support

Restart your iPhone - Apple Support

If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support

Check to see if you have workout power saving mode turned on,this turns off the heart rate monitor.