Damn, that sucks, you don't have recovery questions on that? I've been hacked 3 times before and luckily the first time I got hacked it was my friend who gave it back and told me to put recoveries. You should put recoveries on all your accounts because the chances of getting hacked in this game are pretty high compared to others.
1. You are using Windows, and got a keylogger. You were stupid enough not to have antivirus, and you downloaded something untrusted.
2. You tried to play RS3 Beta. (lol)
3. You told someone your password.
4. Your password was so insecure someone cracked it or guessed it.