I've had something like this happen in the past. It should just need a reset, if you haven't tried that already. Plug it into the charging cradle and hold the home button for 7-10 seconds to reset it. Turn the bluetooth connection off from your phone while the Fit is off. When the Fit restarts, you should be able to turn bluetooth back on and reconnect it.
If you have tried a full reset, then I'm sorry.. no other ideas. I wish you luck.