I'm sorry about the issue. Is your email currently working on another computer or tablet or smartphone?
Do you use Safari application?
Can we do a remote support session, so I can troubleshoot the issue directly on your computer together with you?
Please reply at the box below. Thank you,
Does it give any error or message when signing in your yahoo email account at the yahoo.com website?
Also, can you please click on the Apple logo / menu, then click on About this Mac..., then tell me your Mac OS X version, i.e., 10.6, 10.7, ..., 10.10, 10.11?
Please reply at the box below. Thank you.