The sad news is that if were using the "Add New" on the main page for wordpress.com here http://wordpress.com/#!/post/ then the draft is lost and cannot be recovered. That link is only for creating "Asides":
Aside – These are brief snippets of text that aren’t quite whole blog posts. Useful for quick thoughts and anecdotes http://en.support.wordpress.com/posts/post-formats/
The good news is if your created the post on your own dashboard there may be a happy ending.
Go to Posts > All Posts, hover over the title of the post, click Edit and check the Post Revisions module for previous saved versions of the post.
If you're not seeing the Revisions module while on the editor screen, click Screen Options (top right corner of your Admin page) http://en.support.wordpress.com/screen-options/ to activate it. Then select the post and page modules to include them. http://en.support.wordpress.com/modules/#list-of-modules Then you ought to be able to go here > Dashboard > All Posts > Posts and locate the post and recover it from post revisions.
If you cannot locate the post for locating and restoring a post revision, and it's not in your Trash folder http://en.support.wordpress.com/trash/ then you will not be able to recover it. http://en.support.wordpress.com/trash/#restoring-from-trash
When you start a new post, add the title, set it to private and then publish it. When you are done with the post, set it back to publish and update it as justpi has said. The other choice is to get and use an offline blog client such as Windows Live Writer (Windows) or Ecto (Mac) and then everything is saved on your computer and when you are done with the post, you actually publish it directly from the blog client software. See here for offline blog editors so you don't have to go through this again http://en.support.wordpress.com/xml-rpc