By default the front page of your blog displays all posts (not pages) in reverse chronological order, with the most recently published post on top. The 404 (page not found) error message on the front page of the blog appeared because the Admin of the blog deleted the placeholder Hello World post. http://en.support.wordpress.com/not-found/
If you attempt to create a static front page and fail to create two pages ie. one for your static front page and one for posts to display on then you get the same result.
If you do not want all the posts to show on the front page, then you can create a static front page called for example “Welcome” for your site and a “Blog” page for posts.
To do that create two pages > http://en.support.wordpress.com/pages/ After you do that you go to > Settings > Reading and make the designation change and click “save changes”.
Front page displays
__ Your latest posts
A static page (select below)