If you did not sign in to yahoo for at least three month, then the account is temporarily closed. If you know your ID and password try logging on again and follow the instructions. You will be given two options - one to pay for yahoo services and the other will allow you to log in, but you will lose all you stored files and inbox mails. I've had this happen to me several times. I never took the payed option, but now I try to log in at least once in two or three months.
Hope this helps!