In case of a hacked account send a mail describing your problem to firstname.lastname@example.org. If this doesn't help, call Yahoo: 1-408-349-1572, or 1+
866-562-7219 then press on #2 explain to a customer service person what happened. They will ask you to do certain things and then they compare your original info with the changed info. Within 24 hrs you should get your account back!
Send an email to Yahoo Account Security Department account email@example.com mentioning the problem.
Just keep in mind that you have to be polite with your writing.
1. Provide them the details of the entire story.
2.Mention the time and date when you accessed your account for the last time.
3.Give the password that you last used to accessed your account.
4. Most importantly provide the all the information you entered when you register your account or just the information when you last updated your account.
5. Wait for the response, do not write several times for they will consider your messages as abuse and harassment. Be patient for you are not alone in this situation.