WordPress Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request

Comment on Technical Support by John.

I have over 1400 Posts on this site, & when I installed Stallion, all I can see from the home page is the first several lines of each post, which I understand. However, whenever I click on the post heading or to read more all I get is :

The requested URL /will-apples-siri-be-available-for-your-older-iphone/ was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

When I go to admin and dashboard and select view post I get that error again, BUT when I go to edit post I can see it all.

What happened? And how can I fix this?