This would only happen if you had previously synced your Zotero database with another account. When you then went to set it up with this new account, it would have given you this message:

This Zotero database was last synced with a different account (‘[old username]’) from the current one (‘[new username]’). If you continue, data associated with the ‘[old username]’ account will be removed from this computer.

You would've then had to check a "Remove existing data" checkbox to proceed, at which point it would indeed have deleted your existing local data.

If you hadn't been syncing with your previous account, you would need to restore from a backup of your Zotero data directory — an export isn't a proper backup, though it's not clear what you mean by "exported database backup". (A Zotero RDF export would preserve your collections, but another file format wouldn't, and you'd still lose links to your items from citations in any word processor documents.)

If you had been syncing with your previous account, you could retrieve your previous data by simply setting up syncing for that account.