This thing already happened to one of my friend before and after doing some research and asking questions to other people who have been in crypto for too long here are possible reasons for getting hacked. You opened a fake or a phishing site. You might have also opened some malware while doing or clicking some sites, a key logger must've been installed into your computer. And here is one scary thing that I have heard from one of my friend, it is possible that there's a back door on the contract of the token since you have said that you have no ethereum to transfer the tokens.