Can i ask for a re-roll ?

You don't need to put in a normal support ticket. Just click the "choose another winner" button in the giveaway list.

As for the rule, if you can't contact the winner/winner will not respond to you after a week then it's a valid reason to choose a new winner.

Exactly, don't send the key blindly to his email.

Email his and ask him to send a friend request to you. If he never replies then you have already gone above and beyond in your effort to give him a free game and the game should go to a more grateful winner.