I like many others have been locked out of my yahoo account that i have had for 10 years. I have never changed the username of password. At this point i don't know if someone has hacked into my account or if its some yahoo tech probelms.
Just wanted to share what I have learned and others have had success with.
You can call customer service at 408.349.1572 but they offer little help.

They tell you to send an email to:
[email]account-security-help@cc.yahoo-inc.com[/email]
subject line: your user id

Explain the issue and they will send you a questionaire to fill out.
I am in the midst of this process but know that others have had success and I wanted to put all the information on resolving it in one place.
Good luck and if anyone else has advice please post.
I also posted a complaint with the BBB.