what happened to my chickens?

[old nurse 2]

We have a fenced in yard, we were out for two hrs. returned, gate to chicken yard open. All chickens up in house since it was nigh t Next AM one chicken dead , other three very lethargic, wouldn't eat or come out of cage, then another chicken died, one we gave small amounts of lettuce to pick ate after 4 days finally moved their heads, but so weak. Cones were turning pale to white color. this was a week ago, still no noise from chickens and the y still don't want to be handled, They are eating & drinking now. Did someone or something scare them to death?

 

[ChirpyChicks1]

Sounds like they got into something. What is in your yard as far as plants? Any treatments on the grass?