It's difficult to say what happened with the post disappeared. When you click on Preview, the screen refreshes and there is a preview of how it will look just below the area where you are typing in the content.
If you had clicked on "Save draft" this would save it to your account and you would be able to find and resubmit this post. If you clicked on Preview and it disappeared there's unfortunately no way to retrieve that information.
If you are typing a new post and are afraid that it may be lost, you can select all of the text and copy (control-c on a PC, or command-c on a Mac). This will store the text temporarily and you can then paste (control-v, or command-v) it back into a new post if all is lost.