I can't really help here, since Ray knows a lot more about this than I, but 1) if you have a premium account with Yahoo they should be able to help, since it's unlikely (isn't it ?) that they have completely removed all emails from their system yet, even if they were 'permanently deleted' by someone, and 2) that brings up the question of whether someone who doesn't know much yet about computers could actually have performed the detailed maneuvers of (a) selecting all emails in all folders, (b) moving them to the trash and (c) selecting all emails in that folder and deleting them permanently, all the while "not knowing what she did."
IOW, I realize that any explanation must account for all emails in all folders getting (re)moved, but even if all your emails were sitting in one folder (the inbox ?) I think Ray must surely be correct that possibly they were simply somehow moved to another folder (trash ? spam ?). At least I hope that's all that happened !