question

Contact they or Look at they library if they activated the key and report it or anything else

read the faq, then sacrifice a virgin at a crossroads.

I'm afraid this won't help in your current situation, but for the future you may want to add your winners on steam first, and send the keys via steam chat only after they accept your friend request. Also you can tag them with stuff like "sent key, waiting for them to mark received" if you have a bad memory. :)

If you want to send emails, the best way I find to do it is to send "Congrats, you won X game on SteamGifts, please respond to this email and I'll send the game". If the email is not used, then you lose nothing, and they also can't get the key and pretend they never got anything from you in the case of a ****hole.