Maybe one of thses suggestions will help.
Tap the download and see if you can pause it. Then tap it again and see if the download resumes.
Reboot your router. Unplug it for about 30 seconds and then plug it in again.
Reboot the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the buttons.
Make sure that you do not have a another stalled download in iTunes - a song or podcast .... if you have a download in there that did not finish, complete that one first. Only one thing can download at a time on the iPad so that could be what is causing the problem.
If that doesn't work - sign out of your account, restart the iPad and then sign in again.
Settings>iTunes & App Store>Apple ID. Tap your ID and sign out. Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button.
Go back to Settings>iTunes & App Store>Sign in and then try again.
If all else fails, download the movie in iTunes on your computer and then sync the iPad.