This is a well-known issue with Fitbit specially Charge5. I have had this issue couple of times first time it was under warranty so it was replaced by Fitbit but second time warranty was over and they didn’t do much. I spent like an hour chatting with the guy who asked me to repeat all the steps which I already had but there was no solution, here is a summary of what I found and how I was able to fix the black screen problem, whether it happened during charging just suddenly or during/after an update( this happens more resulting from an update).
1- try the hard reset method through the charging cable button( you can easily watch a video on YouTube to see how to reset through the charging cable)
2- if hard reset doesn’t work try to change the clock face on Fitbit. This, of course, only works if the Fitbit is still syncing with your phone, while the screen is black. (Which is what both of my cases were). Once you change the clock face on Fitbit, usually it would start showing the screen and come out of the black screen.
3- if this also doesn’t work, the only other method left is to let your Fitbit battery totally die and then put it to charging . this will, of course, result in a hard restart. This work for me, but no other method did.
If none of these methods work, I think then the watch needs a replacement.
This issue is so common but unfortunately Fitbit doesn’t have any better answer and I suspect they do not share the feedback with their developers either.
I hope this helps
