If it happens once, don't worry about it too much. Obviously look at things, but don't obsess about it. If it happens again soon, look a lot closer and consider a necropsy. Your county extension agent should be able to tell you where to get one. In some states it is pretty inexpensive and in some, it is not. If you take one in, it needs to be immediately frozen or taken in immediately.
I've had that type of thing happen before. One is dead without a real clear explanation of why. Sometimes I suspect she flew into something on her way down from the roost and broke her neck or something like that. If it is sudden, it could easily be a heart attack or stroke. In some cases, I have no possible explanation. I just don't always know what happened.
If you can smell ammonia in there, I'd give real strong consideration to cutting out a large area at the ceiling-line on an inside wall away from that partially covered window and cover that with hardware cloth to keep predators out. Many of us say chickens can handle cold pretty well, and they can, but I always try to add, IF they have good ventilation.