What in CSS would cause page content to disappear when screen is sized down below 1024px?

I have a client site with an extremely large amount of CSS...Whenever the window resizes below 1024px, the content disappears. As soon as the screen gets above 1024px in size, the content reappears and acts as it should. I'm hoping that somebody can give me a clue where to start looking. I have not seen anything like this before.

I'm not even sure where to start looking. The CSS is not Bootstrap or any other framework. Any suggestions appreciated!