Unfortunately, if you don't know where your backup is, then there probably isn't one. Which means you should immediately take steps to start backing up your laptop so you can recover from things like this in the future.
In the mean time iTunes support may, and I stress the word may, be able to help you recover your movie. On the main screen of the iTunes Store scroll to the bottom and in the "Help" column, click "support." On the resulting web page scroll to the bottom and click the link to "contact support" and proceed from there.
I want to stress, again, the importance of having a current, automatically updating backup scheme. Apple makes it simple to backup your data by providing you with Time Machine built right into OS X. All you have to do is provide an external HDD with enough room. I strongly recommend that after you initate a support ticket with iTunes Store Support you investigate buying an external HDD for use with Time Machine and start using it ASAP.