It's a shame because it is a missed opportunity to learn but I accept that it is not something everyone can get their head around. I wish you and your remaining flock well.
I'm so sorry you lost your hen.
I would second getting a necropsy done or if you are not too squeamish, you could cut her open yourself and take a look. My guess would be that she was suffering from either salpingitis(infection of the oviduct) and/or internal laying/Egg Yolk Peritonitis which...