This is an old thread, but the problem still appears. My forerunner 935 / Garmin Connect on my Android phone had the same issue out of the blue two weeks ago. What solved the problem for me was:
- On the forerunner, in Settings/Phone: disconnect from the phone
- Shut down the forerunner
- In Garmin Connect / Devices remove the forerunner (I've also tried without first removing the forerunner from the devices list, but this didn't solve the problem - for me this step was necessary)
- Uninstall Garmin Connect from the phone
- Restart the forerunner
- Re-install Garmin Connect on the phone
- Pair the forerunner and Garmin Connect; choose to import old training sessions and data from other devices
After that it's tracking HR and sleep again (but it does not show the data from the period when it wasn't working).