This is from the Yahoo Mail Blog :

Due to an unplanned power equipment issue at one of Yahoo's
third-party co-location facilities, a small portion of Yahoo! services
and features may be temporarily unavailable. Our provider is working
expeditiously to complete the maintenance, which we expect to last a
few hours.

What this specifically means for Mail users is that some of you will
be unable to access your Yahoo! Mail for several hours this evening,
and others will experience delays in email delivery. Additionally,
some users may notice that they are unable to login to the chat
feature within the All-New Yahoo! Mail. Overall it is a very small
percentage of users who are impacted, but that doesn't make it any
less frustrating for those of you who are, and for that we are sorry.

Once the maintenance is complete, those users that have been affected
will have full access to their email accounts. Again, we apologize for
the inconvenience and will be working to resolve this as quickly as
possible.