I have lost 4 hens in the last 2 months for no apparent reason. 2 in the last two days. They seemed perfectly healthy up until they weren't. I saw the last one an hour before I found her and she seemed normal. I don't know what is going on. I did have them wormed at the beginning of Jan.
There is a phenomenon that called sodden death syndrome, it could be also related to a fatty liver syndrome, there are a lot of reasons that could cause the un expected death of a chicken.