Have they been eating? Are they showing any signs of labered breathing? While they may have an underlying disease, birds that are sick will stop eating, and that will bring them down even more quickly, especially when they become dehydrated.
Have you done any prevention for your birds?...like treatment for canker, coccidiosis, worms? Have they been innoculated for Paratyphoid? Is your loft weather/wind/predator proofed and all openings larger then 1/4 inch closed? Do you have an rodents coming inside your loft?
You need to isolate the remaining sick birds and get them on heat. Follow these instructions first.