Winner hasn't redeemed/seen? key after one week. What do?

Ask support for a reroll.

Copy/Paste from FAQ:

What should I do if the winner of my giveaway has not redeemed their gift, and I'm unable to get into contact with them?

If you've been unable to reach the winner of your giveaway using email and Steam after one week of your giveaway ending, and they have not yet activated or redeemed any keys or gifts you attempted to send, please contact support to request a new winner. When creating a ticket, we ask you to include data that suggests you made an adequate attempt to contact the winner, such as screenshots of emails, friend requests, or gifts pending on Steam. If the request is approved, a new winner will be generated by the site.

The winner has redeemed my gift, but not marked it "Received". How do I proceed?

If your gift has been activated or redeemed by the winner, and they have not left feedback after one week, please create a support ticket to request received feedback. A support member will review your ticket, and if the gift is activated on the winner's Steam account, they will update the feedback to "Received".

Try to contact him through Steam or e-mail. If he hasn't responded even after that, screenshot your proof of contact (friend request that hasn't been accepted/email message) and request a new winner providing that as proof. This happened to me once too and that's what I did.

Ok, thanks everyone. I added him on Steam, sent an email and took screenshots. If he doesn't respond in any way, I will request new winner tomorrow.

Read the FAQ and you will be fine.

Ok, winner just marked the key as received. I am now level 4 yeah!