Sorry for your loss... As AustralorpsAU stated, she could have been egg bound. It is quite common for poultry to die without explanation as chickens can be affected by disease/stress and not show any symptoms. Was you hen at the bottom of the pecking order? Silkies have rather fragile skulls and a single blow to the head (whether it's continuous pecking or a fight) can result in brain injury. They hide in egg boxes, dark areas or corners if they are being beaten up by flock mates. Though this possibility is unlikely it may have been the case.
Otherwise a more likely cause could have been an acute form of disease such as acute fowl cholera. It is common for Cholera to infect a bird and when acute, not show any symptoms before death. This can happen with a variety of viral/bacterial/protozoa diseases. Otherwise she may have been poisoned due to ingesting rodent baits or possibly avocado leaves or a large amount of grapes etc. Without necrospy it is hard to determine why your hen died. Heat stress can also cause death to a healthy bird if it was hot when she died.
