Unfortunately, this is a known issue. Copy the post in the reader just in case the post doesn’t show on your Posts page, not even in Trash. If it does show there, just edit it, make sure all the content is there, and UPdate or Publish it. If not, paste in the copy you made from the reader.
Also: avoid use of the New Post button on the admin bar or on the “new dash”. Always go to your dashboard and then to Posts->Add New, which seems to minimize this happening.