Each question has a 7-day open period during which people can post answers. A question is closed to additional answers when a best answer is selected or when it is put to a community vote for the best answer. Once a best answer is determined, an Open Question becomes a Resolved Question. Even after the question is resolved you can continue to contribute to answers by commenting on and rate the best answer.
Questions that are open to receiving answers are listed in all categories under “Share what you know. Answer open questions.”
Also, you may not answer your own questions.