It might be coccidiosis at that age. Here are the signs:
lethargic
feathers fluffed up, acting cold
won't eat or drink
stands in corner by itself
may or may not have bloody poo or diarrhea
very quick killer of chicks usually under 8 weeks of age- death can be within a day
chick may or may not have been placed on soil (can be transmitted on shoes, hands, etc. from soil outside)
Treatment is Sulmet or Corid (amprolium). Corid will treat more strains of it. Sulmet is in poultry section of feed store, Corid in the cattle section. You can search on here/google for dosages if needed, or ask. Sometimes you have to treat again if it comes back.