The inbox,sent mails and all the other data folders in my yahoo mail account has been deleted, not from my pc. Some unknown person must have hacked into my account from an external system. How can find the person who did it?
You *might* be able to obtain this information from Yahoo, although they might require law enforcement to get involved. A company like Yahoo has privacy laws to follow, even for people that violate its terms of service by hacking others' accounts.
Those kinds of records have to be released by Yahoo, and they will usually require something like a subpoena or court order. You *can* get these yourself, I believe, but you'd have to hire a lawyer.