I have my iTunes library on my iMac kept on an external drive.

When my iMac falls asleep to save power, the AppleTV can't wake it up to access iTunes. It hangs on the 'Loading' screen or doesn't see the library. To fix it, I just have my iMac set to never go to sleep (only turn off display).

Then I realized that my external harddrives go sleep when not being used to save the drive and power. Again, AppleTV can't wake them up so it hangs for ever. I have to click on any media file in iTunes on the iMac to force the drive awake. Then the AppleTV sees it perfect. I could set my harddrives to never go to sleep but I think I'll just remember to click on something on the iMac before I want to watch something on the AppleTV.

I'm up an running great with my new awesome AppleTV but I just wanted to share what I found in case anyone else sees the same issue.