Why has access to my yahoo account suddenly stopped

It's probably yahoo. they've been upgrading their servers, which means that people experience randomly persistent outages.

Give it a day or so. If it doesn't come back, try deleting and then re adding your account info.

Until it comes back you can do a work around. access yahoo via mobile safari and access your mail that way, It should work regardless of their server work.

I also been having this problem on and off as of March 19. My iPhone and iPad work with no problem, but new mail (from my yahoo account) doesn't download to Mail on my Mac Book Pro (Mail v. 6.3 OS 10.8 Mountain Lion). I have deleted the account several times and also have tried rebuilding the mailboxes, but nothing seems to work, then suddenly all the mails will download, but later on the problem resumes. Right now my last mail that did download was from March 29 (2 days ago).

Any ideas?