You may be able to recover an earlier post revision. 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. http://en.support.wordpress.com/posts/post-revisions/

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

Did you create your post on the dashboard of your own blog?
Dashboard > Posts > All Posts > Add New

Or did you create your post on the front page of WordPress.com?
http://wordpress.com/#!/post/

Advice:
Create your posts on the dashboard of your own blog
Dashboard > Posts > All Posts > Add New

Or use an offline blog editor so you always have backups on your own computer http://en.support.wordpress.com/xml-rpc/