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I would try restarting your phone and then restarting your fitbit using this procedure:
https://help.fitbit.com/articles/en_US/Help_article/1186/
Give this a few tries as it doesn't always work. If that doesn't fix things then there are more troubleshooting ideas in this help file: https://help.fitbit.com/articles/en_US/Help_article/1866/
Note that fitbits can only store detailed data for up to 7 days between syncs. After that it starts to hold data at a daily summary level. After 30 days of that it starts deleting the oldest data. So, when you do get it syncing again you will have missing data which you will not be able to get back.