Reinstalling the app on your phone by deleting it and downloading over the air (rather than syncing to the computer) generally isn't a bad idea if something's not working. Toggling Bluetooth off/on or even rebooting the phone sometimes also fixes a Fitbit sync issue.
If none of this works, two non-phone things: the Fitbit app does seem to "call home", so if something's wrong on the Fitbit servers, the app may not appear to work; this is rare, but it's happened a few times to me over the years. When it happens, there's not much to do but wait it out. And, of course, make sure your Fitbit is charged, or it won't be able to sync with anything. 😉
If all else fails, call Fitbit customer service. Hopefully they can walk you through any sort of reset the unit itself might need.
