can’t leave home

i am spanking new at this and i’ve been mucking about with files and trying out different themes, all the while adding posts and writing pages, and generally having just the best time watching the thing take shape –and now all front page links return 404 (except href to the net): Read more…, Categories, Archives, About, all return /filename/ not found on this server.

my WP installation is in a directory called blog, which calls index.php as its home page. i am afraid i’ve either misplaced a file or trashed the index.php file; any direction i’d much appreciate.