There is along standing issue that Staff are aware of. You may be able to recover an earlier Page Revision. http://en.support.wordpress.com/posts/page-revisions/ Go to Pages > All Pages, hover over the title of the page, click Edit and check the Page Revisions module for previous saved versions of the page.
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 Pages > Pages and locate the page and recover it from page revisions.
If you cannot locate the page by using the instructions I provided 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 post or page, add the title, set it to private and then publish it. When you are done, set it back to publish and update it. Another option 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