We use "might/might not" to talk about possible events/actions. For example: He might come home this Christmas (I am not sure if he comes). Or: I might not be able to come today. Might not can be shortened to mightn't.
We also use "might" to ask for permission (whether we are allowed to do something). For example: Might I open the window? (This is a very polite form of asking for permission).
To ask a question with "might/mightn't" you need to put them first in the sentence. For example: Mightn't this be Sandy's house?