Made giveaway and sent the code - no feedback

Wait two more days.

This is an issue for support not the forums

1) check the winner's library, maybe he has already redeemed your key.

2) try contacting the winner again via mail AND steam chat

3) if 2) fails, create a support ticket

Note: since you've sent a key, there will be no re-roll of the giveaway

Is there any way to check the key if its still working without activating it?

Ok, I checked him. He logged on steamgifts 20 hours ago and is playing dota now... probably forgot about gift. Ill write to him

maybe your email got spamfolder'd try contacting through steam
if the winner's public see if they added it,