This article explains why you may not be able to sign in to Messenger after changing your Yahoo! account password and how to resolve this. 

If you selected "Remember my ID & Password" on the Yahoo! Messenger sign in screen, Messenger stores that sign in information until you manually go back in and change it. Therefore, if you change your Yahoo! account password, Messenger will not automatically update it -- you will need to enter your new password manually.

 How to update Yahoo! Messenger with your new account password:

1. Open Yahoo! Messenger.
2. On the main Yahoo! Messenger sign in screen, clear out your Yahoo! ID:
   - Select your Yahoo! ID in the Yahoo! ID box and press Delete on your keyboard.
3. Clear out your old password:
   - Select your password in the Password box and press Delete on your keyboard.
4. Type your Yahoo! ID and new password in the Yahoo! ID and Password boxes.
5. Leave the Remember my ID & password checkbox checked, if you wish.
6. Click Sign In.

• Avoid an account lockout: If you have tried to sign in to Yahoo! Messenger with your old password, wait a few minutes before going through the sign-in process again with your new password to avoid a lockout situation.
  - In order to protect your account, Yahoo! sets a limit on the number of unsuccessful sign-in attempts allowed in a set period of time.
  - If that limit is reached, Yahoo! sends a security message to the account owner stating that the account has been locked for security reasons.
  - This lockout expires after 24 hours, or you can try to unlock your account by following the steps in the article, Receiving an Account Locked Error Message.
• If you are having sign-in problems, but not getting an account locked error message: If you are having problems signing in to Yahoo! Messenger, see Troubleshooting Tips for Messenger Sign-in Issues.