That all depends upon how you "overwrote" it :)
To end up with "Just One "Hello World" Post!" means you are using a different (new) database (that's where all your posts, among many other things, are stored).
So if you do did in fact delete the original database without backing it up first, then yes, you have in fact lost it all.
Contact your host as they may be able to roll back the site to it's original version from one of their backups.