Okay, I think if you actually go ahead and fill in the login form details then you may be properly screwed.
But in future click back. If it's a new post you're making you should find the boxes at the bottom still all filled in.
If you were replying then click 'reply' under that post again and you should find your reply is still there and appears pre-filled.
Then open a new browser window and go to Drownedinsound and use that to Login. Then return to your old session window where your reply/post is and post it. Should work fine.