If you can't access Yahoo IM either your account may be locked up due to someone hacking your Yahoo account.
This happened to me last month and for two weeks Yahoo wouldn't tell me anything about why my account was locked.
Google Yahoo and get their phone number. Just MAYBE you will be lucky and get someone to tell you what is going on with your account.
Good Luck!
Thanks. I can access "My Yahoo" and my calendar with no problem though. It's just email that I can't access.