if you're using windows, i had a problem where the Fitbit Connect Service went wonky and i had to restart it.
for windows 7, go to Start and in the text field put 'Services.msc'. for previous version of windows, go to Start->Run and then put Services.msc. in the list, highlight Fitbit Connect Service and then to the left, hit restart and see if that lets you sync.
or, just try restarting your computer.
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