What should I do? my Yahoo mail can't sign in.. I don't know why...

Any error messages ?

Did you try lost or forgotten password and see if you perhaps using wrong one

Most of the time, if unable to sign into Yahoo Mail, Gmail Hotmail (http://askpcquestions.com/1806-Cannot-sign-into-Yahoo-Mail-Gmail-or-Hotmail.php) or others, you need to reset your password. You'll be prompted for the secret question/secret answer you setup when you created the account.

If you can't remember these, then you can send an email to the provider requesting them to reset your password. You'll have to go through some questions that verify you are who you say you are, then they should assist you via email in the reset.