How awful. It's terrible having chickens sicken and die so suddenly, and you have no clue what's causing it. It requires some serious investigation, and the lives of your remaining flock may depend on finding the cause.
I've dealt with chickens dying from exposure to petroleum distillates, and that is what the symptoms your chicken is displaying reminds me of. Of course, it could be coccidiosis, an avian virus suddenly causing these chickens to become symptomatic, or one of the Clostridium bacterial infections. It's our task to try to narrow it down so we can hopefully prevent you losing more chickens.
I have questions that can maybe help us with clues if you can answer them. Do you have any yard machines leaking fluids onto the ground where your chickens like to hang out?
How about any recent painting, cleaning brushes with thinner?
Insecticide use?
How about car maintenance? Cooling system flushing or oil changes where some has spilled onto the ground were the chickens have been?
Clostridium. How about compost piles where there may be moldy or rotten veggies and fruit tossed. Perhaps the chickens have been digging quite deep and uncovered some rank, decaying material?
Coccidiosis. Has it been warm, wet, and humid? Have you imported any soil or borrowed any garden implements from someone from another locale? Has anyone visited your place that is from another locale that is always working in the soil, gardening or excavating wearing their work boots to your place?
Feed. Is it fresh? Has any become damp? Have your checked the feeders for mold?
There is a clue or two in there somewhere. Maybe a light will come on and you can give it to us.