Ever type a post or private message, and hit submit, and find that your session logged out? It's happened to me countless times. Fortunately, I learned that if the very first thing I do is hit the "back" button of my browser, it brings me back to the full typed post. I was pretty surprised (and thrilled) to find that to be true.
But, you won't be able to post it because you'll still be logged out. So, keep that window open. Open a second browser window, and login. Then go to the thread you want to post in, and copy/paste your post from your first window into it and click "submit", and you're good to go.
I use Firefox. It may not work on all browsers. But, give it a try before recomposing that long message.