I have had this happen on occasion to me, but I doubt the solution is going to help you at this point. Many browsers make use of the 'Backspace' key to indicate 'go back to the previous page', functionally equivalent to the 'Back' button. Usually, when the browser is behaving itself, and the cursor is in a text box, it will respect the backspace key and it's normal use for deleting characters. However, if you move the focus of the cursor out of the text editing box, usually quite inadvertently, and hit backspace, the browser will whip you back out to the page you were on previously. If you realise this is what has happened, it is sometimes possible to click the Page Forward icon at the top of the browser, which may take you back into your editing window. Now, this is not guaranteed to work 100%, but it has rescued me a time or two on various different forums. But, if you start clicking around to other pages, then you lose that link back into the page you were on. As I said, it won't help you now, but it is worth keeping in mind in case it happens in future.
Like Jason has suggested, write any lengthy posts in a different text editor (I tend to use Notepad), then paste it into the forum reply box. Even if you make changes to the text after you copy it, your original is still someplace else, and it's a lot easier to re-edit than re-create. If you just worry about the text part, you can simply add the image links in once you copy/paste to the forum reply box. In my text version, I simply write a placeholder to myself like this:
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Here's a sentence talking about a fancy car.
[Insert link to picture of fancy car here]
Here's the rest of my nonsense.
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It sucks when you get burned by the technology, but it does teach you a certain level of vigilance.
