You still have another chance. I looked at your site and looks like you have an intro video there, but it only shows how to get started with the site, not the background of the site.

Why not create a separate page - "about" maybe - and post a link to it? Be as descriptive as possible about why you created it, how it's useful to people and who you hope will use it to the fullest.

Then, post your link with the title "Show HN: Somevid" or something similar. As long as you have "Show HN" in the front of the title, it will show up in that section of the site.

Remember to be descriptive and try to relate as clearly as possible what you want to accomplish with the site.

Yes, you cannot post the same URL twice. If you had submitted with Show HN, it should show up in the "new" section. It it gets a lot of upvotes, it'd show up on the frontpage. You might want to delete the earlier post and post it again.