Did your hen pass away or is she just very sick? (sorry, your post was a bit confusing, I apologize)
If you lost her, my condolences.
Birds are very good at masking illness. They evolved this way so that they have the greatest chance to evade predators until they are too sick to carry on. Unfortunately, this means that they often don't show signs to us, either, until something is very wrong indeed.
She may have been showing symptoms, but they were subtle or hard to see.
I'm afraid that there isn't enough information here to even venture a guess. There are dozens and dozens of reasons chickens can appear to "die without warning", and without having her here to examine, it'd be a wild guess at best. How are all your other birds? It would be very wise to check them all over carefully... feel them over gently, check their vents to see if they are clear, make sure they are eating well, listen to their breathing for problems, etc.
Depending on where you live, you may be able to get a low cost or free necropsy (chicken autopsy) that can give you answers. If you live in the USA, you should contact your state's agricultural department, or if they have one, their poultry department. If you want to do a necropsy, here is some information on how to do it:
https://www.backyardchickens.com/t/799747/how-to-send-a-bird-for-a-necropsy-pictures
