If someone has hacked your account there is nothing that you can do without the help of Yahoo.
In both instances you will have to give the Yahoo support team all of your details so that they are sure that it is you that they are dealing with, otherwise anybody could pretend to be anybody in order to get into other people's accounts.
This means that you will have to successfully answer the secret question that you were asked when you first registered with Yahoo.
If you have no emails in your account that you desperately need to access, it may be easier to open a brand new account. This does not take long to do but you need to consider how time consuming and awkward it will be to notify all your contacts that your address has changed. In fact, if you choose this route, you should think about how you are going to get the email addresses of all of your contacts.
Whatever you choose to do, whether it is to wait for the Yahoo support team to sort the situation out, or open a new account, take extra special care that nobody else can find out your password so you do not find yourself in this position again.