There is unfortunately a glitch. The dev team is aware of it and working hard to try to find a solution. Unfortunately, the best work-around right now is to copy/paste/save your post into a notepad file before you click the post button. It doesn't happen very often but if you spent a lot of time on your post it obviously really sucks if you are one of the unlucky ones.
https://github.com/steemit/condenser/issues/1532
RE: My post disappeared!