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
(If you are referring to a page then see here > http://en.support.wordpress.com/pages/page-revisions/)
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/