One of my friends just got hacked on her PC and her Yahoo account password was stolen. Now the hacker use her account under her name for bad reason. He may have change alternative email, security question, etc.What can we do about that? We need her account back. Is it possible? ps. I tried to google search for Yahoo contact information but I'm not sure where to knock Yahoo door. 
How to retrieve a stolen Yahoo account's password?
password recovery;yahoo
From here:How to recover your Hacked Yahoo! Account:To be able to reset your hacked yahoo  password just send a mail to Yahoo  Security Department   account-security-help@cc.yahoo-inc.com.  always remember to be polite in  contacting them, tell the whole story,  dont blame them for your hacked  account, tell them when is the last  time you access your account, supply  them the password that you last used  to accessed your account, give them  all the information you entered when  you register your account or just the  information when you last update your  account.