Apple TV Home Sharing only works when my computer is on

I have movies and music in my itunes library that I want to watch on my Apple TV. Yesterday I was able to watch them no problem - I just clicked on the computers button on the main menu, and my itunes library appeared. Today it won't work unless I have my macbook open. Once I close my computer, the itunes library disappears from my Apple TV.

Is there any way to fix this?