For the first draft, I wouldn't worry too much about the opening. Think about how you simply need to get your characters to the denouement. What do they need to do? What do they need to learn to get there to make the choices you want them to make? How do they get to where they need to go?
Another question to ask: what are your characters doing the moment the change happens--the moment they receive their first call to action, if you will. Then start working out that scene.
I'm reminded of Stephen King's It. It begins with Georgie getting obliterated by the clown... which is the beginning of the story for Bill Dunborough.
If still _nothing_ comes, and this has happened to me, I just start working on a scene that I do know about. Usually I get ideas about how the characters got there, etc, etc, and then I can work backwards.