In addition to what Rhodiegal typed for you, could you please answer the questions in the second sticky of the forum - but answer them into here? that will help us start a flock history so that we can give you more accurate answers and help you better.
https://www.backyardchickens.com/forum/viewtopic.php?id=3569 is the thread.
In addition to those questions, please provide any and all information (even what seems silly) concerning your entire flock. Sometimes the most seemingly insignificant puzzle pieces are those that solve the mystery!
We'd like to help you solve the mystery, indeed.
In the mean time, I'd add these question:
Could the birds have gotten into any maggots, even those just in decaying or wet vegetation?
Any algea puddles, stagnant water, standing water near animal manure piles?
Do you use septic water or city water?
As Rhodiegal suggested, could the birds have gotten into any wet/soured grains or feeds - or ones that had been wet and dried?
Any access to a compost pile or kitchen scraps?
Any access to toxins or weed killers? Or water where toxins might have flowed down from neighbor's houses or the road?
If you grab the feathers near the neck, do they come out easily? (A sign of botulism).
If you feel it's botulism, flushing quickly with a molasses flush or epsom flush can solve the problem. But as both treatments can also dehydrate and stress a bird (and possibly kill them) it's such a last-ditch effort treatment that I rarely ever recommend it, particularly on very little information.
Be sure particularly to check your birds at night several times this week to rule out mites. Mites (and a number of other things, not all parasites) can cause these exact symptoms. however, because they're nearly microscopic and don't always spend time on the bird, they're very hard to catch. So check every inch of your birds especially in the warm/moist areas around the vent, under the wings, etc. Checking at night - when they sometimes come out to feed - over a light colored pillowcase with a flashlight (with the coop lights on) helps to catch them. IF You find them, ask us about treatment as you must treat and retreat the birds and the premises as well.
Be sure to check their weights when you're examining them - how heavy/skinny they are is very important. Also look for reddened vents, anything unusual. Let us know what you find.
Also, do you know if any of your birds were vaccinated for Marek's?