If you distributed the key via the site (entering it at the giveaway creation time) then the winner has not seen it. (You can confirm by the absence of a green eye beside the key in the winners page.) Unless you used the key yourself, it should still be available.

If you sent it to them directly, then the key could be compromised. You can always try opening a "Request received Feedback" ticket in that case and we will review the evidence.