Dead Hen?
Just bring her in for a postmortem examination - free of charge. Fresher the better!

External Parasites
Lice are flat yellow and fast moving, and usually seen around the vent or under the wings. Louse eggs look like clumps of granulated sugar attached to the base of the feathers; it is usually worse in the autumn and winter. They are not life threatening but can lead to depression and decreased egg laying.
Red-mites live in the coop during the day, coming out at night to suck the blood of the birds. Control is based on removing them from the cracks and crevices of the woodwork (they can live for six months with no food)

Internal parasites - worms
Worms are not always visible and infection often does not cause any obvious symptoms. However they can affect growth and egg production, and they can cause general poor condition, anaemia, and, in severe csaes, death. 
Contact us to get  a suitable wormer that can be added to the food, and you can still eat the eggs!

Feather Pecking
This usually occurs around the tail, but it can occur on other parts of the body such as under the neck or under the wings. Often it is one bird that is doing this, and this may be the only one with all her feathers intact. She will particularly like the young feathers which are blood filled, and these will therefore never get a chance to grow.
There are some effective anti-feather pecking sprays on the market, but this will have to be used daily.

Egg Peritonitis
This is probably the most common condition affecting individual laying hens. It may cause sudden death or the hen may stop laying eggs, have  a swollen abdomen, and possibly  a creamy discharge from the vent. Early treatment may bring about  a cure, but generally the outlook is poor.

Moulting
This is a normal occurrence that usually takes place in the late summer or autumn and can last for three to four weeks. Unfortunately egg production usually stops at the same time.

Acting strange!
Is she spending more time in her nest box ? Maybe become a bit aggressive? Not eating much? She's probably gone broody.
If you can, separate her from the others, and put her somewhere where there will be no comfortable nest-box to sit in. It may take a few days for her to get back to normal.

For more information click here