If you need to reset your mystery key which you lost, by then you should go to the sign-in Helper tab. As of now, you should enter your email address, if you don't have a flexible number and tap on the Continue elective. If there is any CAPTCHA, by then fill it. By and by, you will get a check code. Thusly, you can reset your mystery key. If you really fronting the same, by then use Yahoo customer service.