There's an issue with creating posts on the front page of WordPress.com ie. the auto-save does not work. Do not create posts here > http://wordpress.com/#!/post/
Instead I recommend that you ONLY create posts here > Dashboard > Posts > Add New
If you are not running one of the browsers and versions listed at http://browsehappy.com/ please try upgrading or switching first.
Note: If you are using an IE9 browser in compatibility mode that makes it render as IE7 and IE7 is not supported. To disable see > http://www.sevenforums.com/tutorials/1196-internet-explorer-compatibility-view-turn-off.html
If you are, please do these four things:
1. Try clearing your browser’s cache http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=32050
2. Make sure that you have third-party cookies enabled http://en.support.wordpress.com/third-party-cookies/
3. Try with all browser extensions or add-ons temporarily disabled.
4. Try enabling HTTPS: http://en.support.wordpress.com/https/
If you created that posts using the "Add New" on the main page for wordpress.com here http://wordpress.com/#!/post/ then what you describe can happen and worse still the draft posts may be lost and may not be recoverable.
If you created the post in your dashboard here > Dashboard > Posts > All Posts > Add New then 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