Go to Pages > All Pagess, hover over the title of the post, click Edit and check the Pages 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 both 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 Pages > Pages and locate the page and recover it from page revisions.
If you cannot locate the page for locating and restoring a page 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 page or post, add the title, set it to private and then publish it. When you are done with the page or post, set it back to publish and update it. The problem with apages and posts disappearing when saved as a draft has been around for a while, but only seems to hit a small number of people randomly.
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