I had the same trouble. Mine is only 3 days old. I did a restart on my phone, and it is working again!
Restart phone https://help.fitbit.com/articles/en_US/Help_article/1186
Best Answer
@mummatonk wrote:my screen is black, green light comes on at the back, have tried restart but doesn't do anything, seems to sync and can hear it vibrate, less than a week old
Only two things left. Do a Factory Reset back to out of box condition and/or contact Customer Support. If you want to try the Factory Reset, swipe left to Settings, touch About and touch Factory Reset. Customer Support.
Best Answer