Sometimes your apps and clockfaces can start behaving in an unexpected way. Symptoms can be :
App does not respond to settings
App does not display data fetched from the Internet
Or any other unexpected behavior
It is usually caused by a lost connection between the part of the app that runs on your phone and the part that runs on your device.
The fix for this is to just wait, connection will be restored in (more or less) time.
Reset your device (you will not lose data):
Simultaneously pus button C and B. Keep them pushed until the Fitbit logo appears on the display (display will first go black, keep the buttons pushed).
On Versa Lite and Versa 2 press C until the Fitbit logo appears on the display
Restart your phone if the above alone does not solve the issue.