You were phished, I must see a dozen people a week posting on the forums having fallen victim to a phishing scam. The only way your funds can be withdrawn is by using your PIN, most people falling for these scams at the moment have opened up accounts on Atlantis via a phishing link using the same credentials as their SR account. If you didn't do that you should as astor said check your computer for malware, rootkits, keyloggers etc. If you have any of those on your machine you will need to do a HD format and reinstall the OS, they are nearly impossible to remove any other way.