How do you contact people who won giveaways?

That's very unfortunate, hopefully they'll react soon. I'm also replying to your other reply on the comment below: I'd suggest upping your giveaways to Level 2 entry, that could help a little bit if you keep running into this same issue many times. To get to Level 1 people need to give away just anything (that the site doesn't consider to be a freebie), so even a single 1P giveaway will let people become Level 1 without much effort. At Level 2 people have needed to dedicate a bit more effort into giveaways and paying attention to the site, so there's slightly increased odds they'd also be faster in receiving their gifts when they know what it's like to host giveaways. This is naturally not a guarantee and there will still be people you've run into now, but you might want to test it out at some point to see for yourself.

I set them really high (like level 6-7), and I understand that it's not a nice thing to do, I'm just making life easier for myself. I think that a compromise would be sensible: setting level not that high (because if everyone does that, the site will become a mess where the majority won't be able to enter anything), but not as low as you do. So, maybe level 2 or even 3?

i think 3 is a good way to start, you can get to 3 easily, you only need 50 CV, which is about 10-20 bucks
i set my bar even higher