How did you lose them? To a predator? If so, you need to evaluate how it got into your coop and then reinforce that area. If you have a good, strong coop and attached run that has 1/2" hardware cloth covering all large holes, your threat will be lessened. If you free range, you will have to accept the fact that one day something will discover your chickens and you will have losses. Even the most "Fort Knox" like coops can have attacks. Unfortunately, it's a part of chicken keeping. We do the best we can, but some predators can be very crafty and determined.
Your best bet, then would to house them in a coop with attached run, no free ranging. The run should have 1/2" hardware cloth to contain them so nothing can reach in and get ahold of them and no small chicks can get out. Your coop should have 1/2" hardware cloth covering all openings. Make an apron going out from the bottom of the run to discourage digging predators. (Look up user iwiw60 they have posted some pretty good illustrations of what I'm talking about with the apron). You can also put electric fence around the perimeter of your coop. Just know that no matter how tight and secure your coop, you may still lose a bird or birds to a predator. Some of them are very determined and will find any weakness in your setup. In my opinion, mink and raccoons are the worst, along with weasels. And if you live where there are bears, that's a whole 'nother story.