The Fitbit Community is a gathering place for real people who wish to exchange ideas, solutions, tips, techniques, and insight about the Fitbit products and services they love. By joining our Community, you agree to uphold these guidelines, so please take a moment to look them over.
You hve probably accidentally turned off 'Screen Wake'. Scroll right (swipe left) to 'Settings". One of the options under Settings will be 'Screen Wake'; tap to switch between Auto and Manual.
Before posting, re-read to see if it would make sense to someone else not looking at your Fitbit or phone.