It is the 2nd time this behaviour happens so I ought to report it to check if other as well...
I have F5 sapphire just in case it matters. I press down, check the oHR than press up for the watch and get a quick restart/reboot (not sure exactly) with the green triangle on the screen. If I go again to see the oHR, again the restart. If I let the watch in the clock mode than later it works ok. Also the oHR history of the last 4 hours is gone.
It happen to me 2 times in three days.