There’s no way to recover your key. I once accidently deleted mine and faced the same problem. I ended up emailing the few users who had already purchased my app about my mistake and told them to let me know who they are and that I would refund their purchase of the app again, if that’s what they wanted. Since then, I’ve made several copies of my key. Once again, there is no way to recover this.
On the other hand, you could try doing a system restore, on your computer, to a previous date in which you haven’t misplaced or deleted your key yet.