I'd like to apologize for this: I am so sorry.
As raincoaster has said, we are running into an issue with some users experiencing vanishing posts. Can I ask you a few additional questions?
Where were you composing the post? From the "new post" link in the grey admin bar (near your name)? From under the "W" logo in the upper right? From your Edit Post page? From Newdash (the page that has links across the top for Stats, Reader, New Post, Notifications, etc.)?
About how long were you working on the post?
When did you work on this post (Day and time - approx)?
What browser version were you using?
Did you navigate away from the post while you were composing at all?
Thank you for any information you can provide - it will help us chase this issue down.
As raincoaster says, the best way to post for now is from your Dashboard > Posts > New Post page (edit post page).