We use might, may or could to talk about the possibility of something in the present or the future, when we are making a guess about a present situation, or we aren't sure if something will happen in the future.

We use might not and may not to talk about negative possibility. We don't use counld not in the same way as might not and may not. Could not means that something is impossible.

http://www.bbc.co.uk/learningenglish/english/course/lower-intermediate/unit-16/session-2/activity-3