It looks like Yahoo has deactivated/deleted thousands of its users' email accounts on March 1st 2013, and is ignoring all requests for clarification.
I've personally lost over a decade worth of emails even though I'm an active Plus subscriber (which means I pay to never get deactivated). I have also been ignored by their customer service department since I contacted them four days ago.
See below for links to thousands of other users complaining about the same issue:
http://groups.yahoo.com/group/Y-Mail/messages
http://malaysia.answers.yahoo.com/question/index?qid=20130302142839AAiN3Qk
http://nz.answers.yahoo.com/question/index?qid=20130302094306AAK7ivU
Search on Google for "Yahoo Mail account deactivated" and you'll see a huge number of recent posts, all from March 1st 2013.