I finished the story. I reread it and was happy with it. I clicked post and i got brought to the login page because the website had logged me out whilst I was typing. It wouldn't have mattered if I had typed it up somewhere else first and simply copied and pasted, but I had written it directly from my brain onto that forum post so I had no other copies.
I don't know if anything can be done about this. I doubt it can. I'm so fed up though. If a mod could retrieve it somehow I'd be so grateful. If you can't I understand, I'm just feeling a bit defeated now that's all. I know it's probably pointless complaining about it, but that force logout feature after a certain amount of time really screwed up my day.