I try to log in for 3 days and it is locked though it says that the lock will expire approximately 12 hours. What can I do? I try to change my password but I've forgotten the answer of my secret question?

If you have forgotten the answer to the secret question then you are going to have great difficulty in proving that this account actually belongs to you.

Your only option is to contact Yahoo directly and plead your case.

Yes, which is why you ask a question you would always know the answer for.