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That's going to be a problem. Your fitbit only has enough memory to store up to 7 days detailed data between syncs. After that it stores data at a daily level and can do this for up to 30 days. After that it starts deleting data. So, when you do get it going your data will not be complete.
To get it syncing again 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 it still won't sync after that then there are some more ideas to try in this help file: https://help.fitbit.com/articles/en_US/Help_article/1866/