I won a giveaway but the contributor is not online since the day giveaway finished

Just wait. Look in your email to see if the game key is there. Sometimes it appears on spam folder.

Does he/she log in Steam? Perhaps try to contact them in Steam.

Couldn't the GA creator be in a vacation or something? If he/she is from Europe I am sure that could happen.

Try and contact them. The steam 'Last Online' is not accurate. If no response, wait 7 days from the end of giveaway, mark as not received. If you get the game later change it.

Hey, I know you've only been here one month, but I'll go ahead and suggest you have another read of the FAQ. You can't even mark the game as not received yet, since it hasn't been a full week since the giveaway ended, so just be patient, at least for a couple more days.

It's an unfortunate coincidence that you won't be online next week, when the the giveaway creator will probably reappear, but normally you don't lose anything by being a bit late in acknowledging receiving your win.

If you're sent a key, it can't be rerolled anyway, and if you're sent a steam gift, you can just activate it as soon as you're back. Just have another look inside your spam folders and make sure you don't already have a message with your game code in there.

Oh, God, what a disaster!

imo, its purely GA creator's fault. How he should be dumb to create a GA and then to go on vacations...lol.