Think of Lemmy as email. Each post or comment is just an email sent to a distribution group (a community). If your email server goes down, all of those users and distribution groups are gone. Now I'll still have the emails I sent to you in my email box but you won't be able to see them as your email server is offline. Sure you could create a new account on a new server but you'd have to tell everyone about your new address (federate) but there's nothing to associate your old user with your new one and there's no way to backfill data. I could reply-all or forward (comment) on to your new address but there's still no way to associate those old posts with your new account.