Had that exact same thing happening to me a week and a half ago.
Thought it was my mistake as it was quite late, trying to do a lengthy reply.
Never had the possibility to get that back, last weekend when i saw the instruction here how to do that, 2 other posts were in the "draft buffer".
So i quit for that day and replied (much shorter) the next day.
I guess that's another error for the admins to sort out:
After typing a (long) reply, clicking "Post" will get you an empty edit screen telling you you want to create a new thread.