It happened to me too!
Not sure if it is a glitch or the second back up STEEM chain that it gets lost in?
My recommendation now is after you are half way through writing your post...
In another tab, have the Submit Story open as a back up just incase this happens again.
I was able to submit the post the second time since I have heard other people randomly having the same issue.
Sorry about losing your thoughts and time, hopefully you can still be inspired to remember and rewrite it!
Keep STEEM N ON,
Frank
Fortunately, I write in Word and only do clean-up, pictures and links in the Steemit editor.
Do you know anyone else this has happened to? I'm sure that the developers would love to know if there is a common thread.