Samsung galaxy s3 black screen issue?

hi i have a Samsung galaxy s3 which is about 10 months old i haven't had any problems with it before i have only dropped it once and it was fine no scratches. and it hasn't ever been in any contact with water. about a month ago my screen just went black all of a sudden and then when i press the unlock button i know its working as the buttons on the bottom light up and when i touch the screen it makes the noise. i took it into Vodafone(my provider) and they sent it off a few days later i got it back it was fine for 3 days then it broke again so i took it back to Vodafone they sent it off again i have now got it back and it still wont work. i know it isn't the battery as i have switched battery's with one of my Friends and it still wouldn't work and my Friends did. i have a case for it this is the one i have if it helps

http://www.amazon.com/Samsung-Flip-Cover-Galaxy-Pe...

i have resorted it to factory settings i just don't know what to do any help would be greatly appreciated.

many thanks