All data on my account is gone, why?

If that's the case, then there are two possibilities. Firstly, you could be using a different email back then, so you're logging into a different account entirely. Please check your email address and any other emails you might use to see if you have a Dropbox account there.

Secondly, if the email is correct, then your account could've been deactivated due to inactivity and you should've received an email advising you about this. Unfortunately, this means that the account, and all files within, would've been deleted permanently.