Have you tried to login using Firefox browser to see if you can login in your yahoo account?
Or you can try to install any program that will allow you to login in Yahoo services e.g. eprompter email checker from eprompter.com, yahoo messenger

If you still can't login using another internet application, please go to http://help.yahoo.com/l/us/yahoo/security/ then choose "Account Security" to get help in changing your password or to check if your yahoo login has been hacked or not.