Hi, Your Yahoo Account MAY have been Hacked, this is happening to people all over the Yahoo network and has been been for months.
ANYWAY HERE'S WHAT YOU NEED TO DO:-
You need to contact Yahoo by Email.
This can be done by clicking on HELP at the Bottom Right Hand side of the Yahoo Home Page under Horoscopes.
Click on Yahoo Mail, or whatever one is best for you, you can also click on the arrows for more options, or type your problem in the Search Help box.
Go to the Right hand side of page and click on Contact Customer Care.
Click on Category, page will reload for you to click on Sub Category.
Make sure you fill in Brief Details of what has Happened, click Enter.
Depending on the Category you select and the Sub Category you also select you should then see at the Bottom of the Page for more help Click on EMAIL A SUPPORT AGENT. You may have to click on different ones and try variations.
There will then be a Form to fill in. Fill in the form with as much information as you can remember about your security details and your account.
Yahoo will then contact you at a Different Email Address that you need to give them. This email address CANNOT be a yahoo email one.
You need to give them a different Email Address, it could be one that you already have or you will need to create a new one, Google, Hotmail, etc. You do this by clicking on Email and then click on Register and follow the instructions.
They only need this other email address to give you instructions regarding your yahoo account.
You will be asked to create a New Password, Best to have one that is very hard to guess for anyone else trying to access your Yahoo Account.
I Would Also Strongly advise that you register your Mobile Number/Cell Phone number with Yahoo, Account Settings, that way if your account is hacked a message should come up and they can send a code to your phone for you to then unlock your email account.