Go to Dashboard > Posts > All Posts there 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/)
If you cannot locate a revision or restore from Trash the content has been lost and cannot be recovered.
Always create your posts on the dashboard of your own blog at
Dashboard > Posts > All Posts > Add New
Immediately after creating a post and entering a title click the save draft link. Thereafter, "click save draft" frequently.
Always allow pages to fully load before clicking any other links.
Never leave a post or page open for content creation or editing and walk away from it. Save it properly and return to edit it later.
Use an offline blog editor so you have backups on your own computer