Hen got lethargic and died
Welcome to BYC, though under sad circumstances, sorry you lost your bird. Unfortunately unless you noted any clear symptoms there is not really any way to know what happened to her unless you send/take her for a necropsy at a state poultry pathology lab. It is normal for there to be some fluid that leaks from the beak/nostril when they pass so don't worry about that. Did you notice if there were any abnormalities on her at all, like loss of weight or bloating in the abdomen? Had she been laying regularly?
Have you brought in any new birds recently? Coccidiosis is a quick killer and in adult birds they often fight it until they are very sick and just seem to die suddenly. Aside from that she could have had any one of a number of internal problems that either claimed her quickly or that she was able to hide the symptoms of until the very last. Chickens are experts at hiding an illness until they are very sick birds.