When you upgrade from Yahoo!'s Overture product to the new Yahoo! "Panama" product, the data from your reports in the Overture area will not be transferred over to Panama. This was disclosed in all the upgrade documentation, but there seems to be some confusion about this in the forums.
A WebmasterWorld thread asks why hasn't the data been moved over. It is just the way it is.
If you want old data, you will have to login to your Overture account over here and access that data. Otherwise, you are starting with a blank screen, new data set.
Forum discussion at WebmasterWorld.