My hens were scared when a neighbor's dog got loose and came to our chicken yard. They were protected inside the run but still got very scared (they were pullets back then). They stayed in their coop for 3 days, did not even want to come out to eat so I had to moved water and feed inside the coop. They eventually came out and life came back to normal. If your chickens are young, they are very easy to get frighten, but they will grow out of it after some time has passed.