My Apple TV just keeps loading for th last three hours. What is wrong. All the previews played fine. HELP!

You probably just have a poor connection. I would suggest to stop, unplug Apple TV for 30 seconds, then try again. You can start the download again from the Purchases section.

You will also want to verify the network, there is a big difference between streaming a trailer and a full HD movie

Check current connection via speedtest.net

Make sure DNS is set to auto (settings - general - network)

If on wifi try Ethernet