won a giveaway already a week passed and havent received key

Write him an e-mail or through Steam. Wait 1 more week and then, mark as not received.

Was he/she online? Perhaps giveaway giver quarantined. I believe nowadays everything is possible and in some cases, it could happen much longer deliveries.

FAQ

[FAQ]
If I win a game, when can I expect to receive it?

Giveaway creators are allowed one week to send the gift after their giveaway has ended. Depending on the type of gift and the giveaway creator, you might receive the gift to the email address you have listed on the account page, a key might be added beside the gift you won, or the giveaway creator might have sent you a friend request on Steam to make the exchange. Please remain patient, and do not contact the giveaway creator during this time. If the gift is not received after one week, you can mark the gift as not received, and attempt to contact the giveaway creator on Steam to follow-up.

When should I mark a gift I won as "Received" or "Not Received"?

You should only mark a gift as "Received" when you have both (i) received the exact game described in the giveaway, and (ii) activated that game on the Steam account you used during registration. In all other instances you should mark the gift as "Not Received" one week after the giveaway has ended.

1. Check the login status of the other party
2. If are absent → cannot help → not receive
3. If online → try to contact
4. Online but can not contact → consult support

I think it was like this.

Check here first:
https://www.steamgifts.com/giveaways/won

Please check if you have friend invite on Steam...

As others have said, it’s worth checking if the GA creator has logged in since the win. Leave a message on the GA. And feel free to mark not received, you can change it when the GA creator gets back to you.

mark as not received, then you can try to contact the ga creator and sort things out.