I had a similar problem with my Forerunner 35... recording on the watch itself, but not showing on the Connect App.
The inadvertent solution was - like most electronic goods - to reboot the watch. I did so by running the battery flat. Once it was charged up again the heart rate re-appeared on the Connect App on my cell phone. There was also no loss in the previous data over the last few weeks and months.
thanks for that. I hid it by accident and it wasn't at all obvious how to get it back.
Hope you’ve figured out by now. I’m answering in case someone else has a similar experience and all the other answers are kind of old from few years ago. At the very bottom, the edit my day button, there’s the option to restore hidden cards.