I cannot log in to my yahoo mail ?

Can't log into Yahoo Mail?
Are you sure that Caps Lock isn't on, Upper-case letters and lower-case letters are different and so your Yahoo ID must be entered correctly note how the @yahoo.com part isn't capitalized and the name can vary.

If still you can't log into your account after you have checked all spelling and casing for errors then you can request a new password from Yahoo.

Has your Yahoo Mail account been compromised?

If you notice changes to your account that you did not make yourself and you think that your Yahoo account has been compromised, immediately change your password.

If you are not able to sign into your Yahoo account, visit the Yahoo's sign in problems page. These pages ask for basic information, such as your birthday and the postal code you provided when you registered to verify that you are the true owner of the account. After verification, Yahoo will reset your password.
Next, report the possible fraud to Yahoo. Contact customer care and supply as much detail as possible regarding the problem: Click this link:--- http://schematicsdiagram.blogspot.com/2011/06/how-to-troubleshoot-scams-and-phishing.html

Did you reply to an unsolicited email asking for your Yahoo ID or password, please include as much information as possible regarding the email you received
Do you believe your password was collected from a website that was not Yahoo Include the full address of the webpage you saw collecting passwords.

NOTE: If you supplied credit card or bank account numbers to an unverified website or in an email message, you should immediately report this to your financial institutions.
Immediately after you have either restored your account or created a new one, contact all the people you had in your contacts and tell them that your account had/has been compromised and that they must ignore any emails received from it.