Here are the instructions on how to proceed:
1. Click on THIS LINK
2. As Topic, select Password ***** sign in
3. As Sub-topic, select Forgot my password
4. Enter your Yahoo! ID (e-mail address) on the right and click Submit
5. Under the "We're here to help!" section, check if you can see the "Call us" option. If yes, click it and you will see the phone number you can call.
If you don't see the "Call us" options, then click on "Email".
There fill in all the boxes and explain the problem in the "Detailed description of issue" box. Make sure to enter a valid contact e-mail address in the "Email address that you have access to" so that the Yahoo! support can contact you there regarding the status of the problem.
Once you have submitted the form, wait for their response. It is usually within 3 business days, so please be patient.