I rented a movie in the iTunes Store but now when I want to watch it it says; 'cannot load the movie' while it says the download has been completed.. What now? I already payed for it.
Did you rent the movie on your iPad? Are you trying to watch it on your iPad?
Do you have WiFi connectivity when trying to start viewing the movie?
You should have answered "yes" to all the above. If not, that is probably the source of your issue.