Articles missing front end!

My blog has been working fine for months. Today, the homepage looks fine, but if I click on many of the titles, it doesn’t go to the articles. Instead, I get:

“Not Found

The requested URL /category/name-of-article/ was not found on this server.

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

(category and name of article change depending on the page in question)

I’ve doublechecked articles on the backend, and they have the right URLs. When I hit “view post” I get the same error.

This change is only about 30 minutes old because I just replied to a commenter. I’ve made no changes at all between then and now.

here is the website: http://blog.cnbeyer.com/

My entire site is like this. please help!