Any data that has synced goes to the Fitbit computers database. You can access it from any app, any computer, delete and reload app, as long as you log in with same ID.
Often when you seem to have lost all historic data, it turns out that you logged back in with a different Fitbit ID/email, maybe one you signed up with a while ago, instead of current one.
Before posting, re-read to see if it would make sense to someone else not looking at your Fitbit or phone.
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