It seems that Yahoo! Mail is having server-side issues with users reporting that they can log-in but get stuck with a welcome message which says:
Welcome to Yahoo! Mail
Yahoo! Mail is now even faster, safer, and easier to use. Yahoo! now automatically identifies items such as words, links, people, and subjects from your email to learn what matters to you so that we can deliver exciting new product features and relevant ads.
There are two buttons which you can click, obviously: “I Accept”, and “No Thanks.” Clicking either of these produces no result and keep you with a screen stuck with the welcome message.
No solution has been found at the moment, but if you’re experiencing this specific problem do comment below and report any updates on this matter to help out the community.
UPDATE #1: Some users have reported that using Safari or Internet Explorer 9 allows them to get through
UPDATE #2: Someclaim that clearing their cache and cookies allows them to access their e-mail