I tried to access Fitbit through my PC and got back an error page.

That looks pretty serious. Usually when a server is down for routine maintenance, there is a message giving you the times that the server is down. I hope they didn’t get hacked like so many websites. I don’t know if any of my health data is backed up locally.
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The site is back up now. Hopefully it was just a random error and they weren’t hacked.