Oh! I am sorry to hear this? Do you have a thermometer in your brooder so you can be sure that it is warm enough? Did you say that it does have sides on it? Hopefully it does, because that will help keep the heat in and also keep your chicks from any drafts. I always know when my chicks are warm enough because they are spread out all over the brooder. Make sure that you have enough food and water available so they don't trample each other. Also you can add a tablespoon or 2 of sugar to your water for a couple of days. That can help perk them up.
Hope this help!!!