All articles open in an error 404 page

I just installed the Hot News theme and cannot add an article. The problem is that all Sample articles show on the page but when I try to open them and it takes me to an error page "This page does not exist". On the Demo all articles open in a new page and have the remaining text. The same is true with the articles I add. Also, when I try to open a menu item in Top Menu 2 it takes me to the same error page.

Thank you.

Tru to turn off then on SEF in global configuration