There can be numerous reasons that might be preventing you from accessing your Yahoo Mail account. Below are the most common reasons with solutions.
• Yahoo Mail is having trouble connecting to your internet network. Check your internet connection.
• Using an unsupported or outdated web browser to access Yahoo Mail. To fix the issue, use a supported web browser with latest updated to sign into your Yahoo Mail account.
• Entering wrong account password. Verify your password and make sure the Caps Lock key on your keyboard is not ON.
• Your Yahoo Mail account has been hacked by someone. Reset password for your Yahoo Mail account using Sign-in Helper via link - https://login.yahoo.com/account/challenge/username?authMechanism=secondary&yid=&done=https%3A%2F%2Fwww.yahoo.com%2F&sessionIndex=Qg--
• Yahoo Mail temporarily blocked access to your account due to suspicious activities in your account. Wait for sometime before again trying to sign into your Yahoo Mail account. The problem should get fixed automatically.

Create Signaure in Yahoo mail