You may have clicked on the link in a comment that talks about airdrops, maybe also something about ZERION. If you clicked the link, they then asked for your active private key. That key controls your steem and SBD so they were able to move it to an account that they use to move the money off steem to an exchange where they probably sell it.

You may want to change your password, which will then change all your private keys. Otherwise they may move out more of your liquid assets. I wish you well.