Hi wanda, and welcome to the forum.
I'm not sure I'm understanding exactly what your problem is either, so we'll see if we can pin it down exactly.
I use Yahoo Mail, and I have no problem at all getting into it. Which version of Yahoo do you use? There's a "Classic" version, which I use, and there's the "All New Mail" which I hate.
This is how you'll recognize the "Classic" version from the top left corner.

I ask that, because I've heard of quite a few people having problems with the "All New Yahoo Mail".
Secondly, just to confirm that you have two completely seperate eMail accounts. One with MSN, which is your Hotmail account, and one with Yahoo, which is completely seperate. Am I right so far?
Are you being physically prevented from logging on to either of them when using the correct username and password? Or were your username and password automatically filled in, and are now missing when you access the login screen.
I'm wondering if your talking about the loss of cookie information, or if you're actually being barred from logging in manually.
If I've got this totally wrong, no worries, it won't be the first time, as long as we get to the crux of your problem.
EDIT: Didn't see the above reply until I posted, but I'll leave it, as we both appear to be puzzled.