Have you at any point created an account on Atlantis or any other site with the same credentials? Or have you ever accessed SR from a url that was NOT silkroadvb5piz3r.onion? In order for someone to have gotten into your account and taken your bitcoins, they'd need to know your username, password, and PIN. Your password and PIN should both be strong - this means that the PIN needs to include letters and numbers, not just 4 digits.You can open a support ticket on the main site by sending a message there to customer support.