Can a book start in the middle of the story?

When I started the story I didn't realize I was actually starting it in the middle. It wasn't until I let someone read the 13 chapters of my rough draft that it was brought to my attention. The story isn't done but I'm wondering if I should bother finishing it. Should I finish the story and then work on a prequel or should I stop now and start brainstorming the beginning half and make it all one story?