Flex 2 has stopped syncing and won't pair with phone

[Starter10]

My flex 2 suddenly stopped syncing and now the phone will not pair with the flex 2. I have tried turning off and on the Bluetooth, reinstalling the app, tried to replace device on app, all with no success. 

[SteveH]

First thing to try when a fitbit misbehaves is a restart using the following procedure:

1. Plug the charging cable into the USB port on your computer or a UL-certified USB wall charger.
2. Remove the pebble from the wristband and press it into the compartment on the charging cable. The pins on the charging cable must align with the corresponding pins on the back of the pebble to lock securely in place.
   

   You’ll know the connection is secure when the pebble vibrates and you see the indicator lights blinking.

3. Find the button on the charging cable below the tracker compartment.
4. Press it three times within five seconds. The tracker vibrates each time you press the button. A few seconds after the third press, all the lights will flash simultaneously. This means the pebble has restarted. If you miss the light flash but notice the pebble has resumed charging, that too is confirmation that the restart was successful.
5. Remove the pebble from the charging cable and insert it back into the wristband.
   
   

It does no harm and can sort out all sorts of issues. You might need to give it a few tries though as it's known to not always work.