@nirdesh71671 this "first developed black sections slowly the whole screen went black" sounds like a bad screen. If it would be a data issue it would not have done it in sections. It sounds to me like you are do for a new screen.
Since iFixit does not yet have a guide for that, use something like [https://www.youtube.com/watch?v=qN8XGGpFGfo|this video] to work on your Fitbit.
Screens appear to be available at places like ebay and others.
While you work on your Fitbit, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.
Repair is War on Entropy!