I rented a movie on iTunes, it started to download and now I can't find it. The download tab is no visible in itunes

The Downloads tab only shows whilst you have downloads happening/waiting, if it no longer shows then that implies that the download has completed - the rental should be in the Videos app for viewing.

If you rented it on your iPad then you have a separate Videos app on it, it's not within the iTunes Store app - the Vidoes app's icon looks like a film clapper-board (blue with black and white across its top).

I also started downloading a movie. The download got interrupted and now I can't find it. How do I get the download back to finish downloading the movie?

If it's on an iOS device and it's finished downloading then it'll be in the Videos app. If it hasnt finished then check to see if it's in the Purchased tab in the iTunes Store app for redownloading.