I had the same thing happen to me as others have said. When using electrum a message appeared saying I need to update the app. Then after redownloading I saw my funds had been sent to another account.
Sorry about that you have been a victim of the message being sent to old versions of electrum asking to update your app with their own version of electrum, once you have added your private key to their own version your BTC is good as gone. They have announced it on their website that hackers somehow have been able to broadcast their own message for older versions of their software, I believe you can do anything to be able to retrieve it back as it might be transferred to a dozen addresses by now.