Don’t use the “Pop-out” link before saving your writing. Pop-out is the link at lower left of the wordpress New Post window. It will pop open a new window to post in, and erase anything in the post you’re in. Whoop.
I just found out the hard way, and really, is there any other way? Oh yes, there is. You can avoid losing your blog post as I just did by popping out before you start composing, or copy-pasting what you’ve written so far.