If you are unable to sign in to your yahoo email account, you can try entering the fields correctly again if you made any mistakes the user-name and the password. If nothing works after re-entering the fields, you will be required to fill "Yahoo! ID Search form" about primary details you used in signing up the account which will be used to locate a match. The details will be sent to your alternative email address automatically.
I had same problem. Open up another email address, I used Google. Email Yahoo and let them know what is happening. It was months before it was completely fixed so you may need other email address. They stayed right on top of it till it was fixed. I couldn't get into my Yahoo email at all even on IPhone. Now I can. Good luck.