All, (Updated 18th December 2019)
Like yourselves, yesterday I also experienced my Gear Sport showing a blank screen, however it was still operating as it was still vibrating when notifications came in.
Luckily or coincidentally my Samsung mobile also started playing up at the same time, which indicated that the problem was not with either watch or the phone.
Whereas before I had offered the below advice in brackets;
(I had recently started using a multiple USB port plug to charge both phone and watch off one electrical socket. I had noted that both the watch and the phone had registered as fully charged the day my blank screen occurred and the phone battery discharged itself in just under six hours.
I had read the other articles about rebooting, but none of these helped resolve the issue with the watch. It was when the watch lost power in under 12 hours, that I concluded it must be something to do with the charging. I dug out the original Samsung supplied plug and attached the watch to the watch cradle. Immediately the screen re-appeared, I left the watch to fully charge in the cradle and have had no more re-occurrences of the blank screen.
I was aware of the Apple products being a bit 'tetchy' if users do not use original Apple label cables and plugs, but this is the first time I have witnessed it in a Samsung product.
Before all else try charging the watch with original Samsung plug and cradle ensuring you see the lightning bolt indicating the watch is charging! If you have inadvertently switched the watch off when trying to recover the screen, don't forget to switch it back on whilst it is in the cradle to confirm the screen is back.)
I now realize the problem is nothing to do with the how I charged the watch/activity tracker, it's related to a software patch which Samsung took several attempts at getting right and as a result affected my use of the tracker for several days. More than likely due to preparations for getting the population onto the 5G network!