Well i am sailing in the same boat. I changed nothing in the site, just normal upgradation of site and suddenly on 25th nov whole of the site is out of index and just the index page is there. Pretty weird how only the index page is in the index for everyone. At this time it doesnt seem manual penalty since lot of people are facing this problem but can't ignore that fact also.
Anyone with other thoughts as to why this changes occured in yahoo?
Also can yahoo guy also let us know as to whats going on?