Single Post Pages Not Working?

On a site I am working on (http://LorenPaulCaplin.com) the single post page for posts does not seem to work. When I click on the link to a post it does the 404 behavior (in this case, go back to the index.) The theme I am using doesn’t have a single.php , I tried to create one, though I don’t really know how to make it function, and couldn’t make it work.

Thanks in advance,

Josh