Login problems can come from a variety of sources, from both the user side and the platform. If you can't sign in to your email account, it's possible the service is down or that you made a mistake with your login information. If you can’t access yahoo mail then check the status of Yahoo Mail. If it's down, nobody can sign in anywhere, and someone likely reported it to a site such as Is It Down Right Now. Likewise, check or contact @YahooMail on Twitter to see if anyone else reported a problem. If Yahoo Mail is down, wait and check back until the status changes.