Sorry but almost certainly gone forever. Not stored on the local machine as you type other than as part of the webpage. You best bet at the time is go back one page and refresh the screen but even then it would have depended on your webpage settings.
It you're working on a really long post try selecting all the text and using copy to hold it in your clipboard memory "just in case" but even that won't work if you lost power and hadn't actually saved it to a file.