It may be possible to recover what you lost from an earlier Post Revision as there is an autosave feature. http://en.support.wordpress.com/posts/post-revisions/ In the event you want to try to recover it then here's what you can do. 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 http://en.support.wordpress.com/screen-options/ (top right corner of your Admin page) to activate it. Then select the post and page modules to include them. http://en.support.wordpress.com/modules/#list-of-modules

I had saved, previewed, then move to another Forefox browser tab to read email. Returned to blog post and content all gone.

Sent "help" plea to WP support, but is there any possibility of recovery? It was a huge amount of work. (e.g. Mailchimp has an auto-save function).
Thanks