To access your movies, music, photos, etc. on Apple TV that are stored on a computer you need to enable Home Sharing on both devices.
In order for Home Sharing to work, you must be signed in using the same Apple ID on your Apple TV and in iTunes on the computer with your content. The computer and Apple TV must be on the same wifi or ethernet network and the computer cannot be asleep.
Here is more on how to set up Home Sharing for Apple TV.
Wake Computer for Home Sharing
Apple has a Wake on Demand feature that will wake your Mac for accessing iTunes content. The only problem is you need a Mac and a Time Capsule or Airport Base Station.
If you have a PC or third-party router than this feature does you no good.
However, there are several apps that you can install on an iPhone, iPod touch or iPad that will wake your computer.
Awake is a Free app that will wake your computer from any iOS device. Then you will be able to access to your content on Apple TV via Home Sharing. The only catch is that your computer needs to be connected via an ethernet cable. If that is how your computer is set up, then Awake may solve a simple problem for you.
If you do not use a cable to connect your computer to the internet you may find it easier to just manually wake your computer when using Home Sharing on Apple TV.