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In the hope that this is just a software glitch I would try restarting it using this procedure:
https://help.fitbit.com/articles/en_US/Help_article/1186/
Give this a few tries though as it sometimes doesn't work. If that doesn't kick it into life then I suspect it is defective. Are you able to exchange it at the store you bought it from? This would be a quicker route than getting fitbit to replace it.
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