What happen if you won a giveaway but you don't receive the key?

mark as not recived that all you can do

Contact him.
If he didn't give you the key, Mark as not received.

When it has been almost 7 days, just try contacting them through Steam. Some people forget they have made giveaways. If they dont answer, or say you wont get the game, mark it as "not received"

Dude, chill up. He has 7 days to send you the gift.

We can give away Metro 2033 now?

Try to contact the creator on steam. If no response, wait for 7 days from the day the giveaway ended. Mark it as not received. Nothing else you can do about it.