Quick, write what you have down! The worst possible thing is if you lose your amazing idea for your story. Write what you have down and go from there. If it works for your story, try doing a flashback. How did your characters meet?? Perhaps the first chapter is from someone else, aside from your main character's point of view. Where are they? How did they get there? Remember, you know your story best! Happy Writing! 

I went to a (really totally awesome) writing camp this past summer. Every day, to start off, we would have a "free write." We'd just get out a piece of paper and start writing. Anything and everything, but we had to write. You could write your phone number over and over, your family member names, whatever works. After a few minutes of that, people started having all these ideas. Good luck!