Are you trying to retrieve the password for it, yet it still isn't working?
This actually happened with my previous Yahoo! Email account. I was attempting to retrieve my password after a long time of not using it because I forgot it. When I came to the part where it asked for me to fill out that one code with distorted text, it would always state that I got it wrong even though I was correct. This happened for a while, and I eventually came to the conclusion that my Yahoo! Account might have been suspended. I'm not saying yours is, but that could be a possibility.
If this is the case, create a new email account. If it isn't, then would you mind going into more detail as to how it won't let you login?