For those who are still having issues I have a RTX2070 and had this problem back in December. The following advice Render device lost recently - #18 by Zaze-1399 fixed the problem for me.
The problem popped up again today so I repeated the same process and it seems to have fixed it. For some reason something broke the link between Overwatch and the program specific graphics settings in my NVIDIA control panel that needed to be changed (Overwatch exe now appears twice in my NVIDIA control panel). I’d recommend trying the above fix if the other methods don’t work. Be aware this relies on your Overwatch exe being a consistent name so if you change it you would need to repeat the above fix again.