You may be able to retrieve some of the lost emails via Yahoo's recovery procedure here: https://help.yahoo.com/kb/sln2552.html
Consider setting up 2 step verification to prevent further unauthorised access. Be sure to make and keep safely a separate note of your Recovery Code,. if you do
Be wary of phishing attempts - you probably gave away your login details to one of them unwittingly. It's highly unlikely you were actually hacked.
