This article explains how to fix this when you can't log in to Yahoo Mail.

Login issues can come from different sources, both from the user side and from the platform. If you can’t log in to your email account, it’s possible that the service is down or that you’ve made a mistake with your login details.

Try the following strategies to find out what is preventing you from opening Yahoo.

Check the status of Yahoo Mail. No one can log in anywhere when it's down, and someone has probably reported it on a site like Is It Down Right Now. Check or contact @YahooMail on X (formerly Twitter) to see if anyone else has reported a problem. If Yahoo Mail is down, wait and check again for the status to change.