Missing button on giveaways - won page

It sounds like the giveaway creator went back and deleted the key (might have realized it was duped?). If you can see the received/ not received selections it hasnt been re-rolled. I would try to contact the creator on steam. If you were in a group and got kicked they may have re-rolled it.

1 week ago

Creator could hide the key when it was not used and asked support for re-roll.

in case it happens again, you can redeem keys via browser https://store.steampowered.com/account/registerkey

Sounds like you were re-rolled.
You can mark it Not Received.

Guideline chack?
https://www.steamgifts.com/about/guidelines

--Winning

• Won gifts should be activated to the Steam account used during registration, and they should not be regifted, traded, or sold.
  -Allow the giveaway creator one week to send the gift. Please be patient and do not contact them during this time. If the gift is not received >after one week, you may contact the giveaway creator on Steam to follow-up.
  -Winners should correctly mark their gift as received or not received within one week of the giveaway closing. This feedback should be kept up-to-date if the status of the gift changes.

Perhaps, have you exceeded a week?