That's terrible. Sorry to hear it.
There are many things that could have gone wrong. More info would be needed, including age of bird, size of flock, normal diet and treats, any recent changes, weather, size of coop and run, any abnormal poos.
Sometimes, it just happens.
But if you want to try an identify the cause we can and you might post in the emergencies area.
It's important to make sure the rest of your flock is not at risk. So you need to keep an eye out for any symptoms which might be... not eating or drinking, sleepy, wobbly, weird poos, staying alone... just for starters.
It may be too late now, but if you have more loss... you might be able to get a necropsy done for cheap or free either at the vet, government ag dept, or university with animal science dept. Would probably have to refrigerate the carcass.