Alright I had my yahoo account hacked about a month ago. Heres what you have to do...
First check your alternate e-mail account that you used when you created your account to see what they changed the password to.
Second use the change password utility every 12 hours, and spam it until it says your account is locked. This prevents the hacker from doing anything with your account.
Third call yahoo, (408) 349-3300 it took me about 15 minutes to find that number.
Fourth use the e-mail contact utility, as long as you have all the information that you used when you CREATED your yahoo account you will be able to recover it eventually.