I recommend you to try the following list of troubleshooting tips:
1) Make sure your browser is set to "Accept All Cookies."
2) Clear your browser history and cache.
3) Make sure you have JavaScript enabled in your browser.
4) Make sure any Anti-Virus software and security software are disabled temporarily when you try go back to My yahoo login.
5) Zone Alarm blocks cookies and advertisements automatically. I suggest that you should adjust your settings in Zone Alarm if it is installed on your system.
6) Close the browser.
You can also try logging in from http://my.yahoo.com/. This page may let you log in where others won't.
Still problem persists please send us the operating system and browser version you are using in order to assist properly.
Sam
I can't log in to My Yahoo for the last days. I can log in to my Yahoo mail account, however. Anyone having this problem or aware of what the cause is?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic