why will my rented movie not play?

No idea without more specific information: Mac model, OS version (you are not running iOS on a Mac), steps you're taken within iTunes store, etc, etc.

My first time on discussion forum is about my first attempt to rent a movie on Apple TV.

I rented it a couple of days ago and it took overnight to download. When i went to play it today it shows as "Rented' but has started loading again .. 3 hours 46 minutes to go.

If I downloaded it already why do I have to go through the agony of loading it all over again? I thought once downloaded, it was there until it expired. Just ready to roll.

What happened and what do I do?

Sorry, I've never rented a movie, so I have no answer for you; hopefully someone else will chime in. By the way, it would have been better for you to start your own new thread instead of digging up an old one, but that's ok - you just tend to get more answers starting your own.

Thanks -- I actually dont know how to start a new one!!

Okay, so check the screenshot - first pick a community (I just chose iMac, but there are oodles of different ones), then click on the "start a discussion" in the right column and go from there: