I would like to know how the published version has dissappeared and if it can be retrieved.
If and only if the post was created on the dashboard of your own blog at Dashboard > Posts > All Posts > Add New then you ought to be able to recover a lost post or page from revisions if needs be.
See also if applicable > restoring from trash http://en.support.wordpress.com/trash/#restoring-from-trash
But if the post was created here http://wordpress.com/#!/post/ there is no means of recovery as the auto-save is not functioning there.
Always create your posts on the dashboard of your own blog at
Dashboard > Posts > All Posts > Add New
The auto-save works there and the post will be Standard post format unless you change it.
Do not create posts here > http://wordpress.com/#!/post/ where the auto-save is not working and where you are creating an Aside post format.
Use an offline blog editor so you have backups on your own computer http://en.support.wordpress.com/xml-rpc/