Try these steps:
1. Turn off the phone
2. Press and hold Volume Down Key + Power/Lock Key at the same time.
3. Release the keys when the FACTORY HARD RESET screen appears.
4. Read the warning and press the Power/ Lock Key to confirm. Press any other key to cancel.
5. Press the Power/Lock Key to confirm once more or press any other key to cancel.
6. Your device will perform a hard reset.
I mistakenly wrote the solution of some other request to your question due to some internet connection issue. Try restarting the fitbit flex.
Now give me the status, does it work normally.