[Rebel]

Our one year old ISA brown, Princess, started a mild moult in August/September but carried on laying as normal. She then laid shell-less eggs for a while but stopped laying completely a couple of weeks ago. She has continued to look 'tatty' all this time but otherwise as active and healthy as normal, eating fine, etc. However, over the last couple of days poor Princess has started shedding her breast feathers at an alarming rate. Soon she will be fully plucked...and it's getting cold at night! What could be the problem?

[Roughlee Handled]

She going through a full moult.

Sounds to me that she had a partial moult and now she is in full

There are partial moults and full moults. The latter normally once a year. 

Do  not worry. I had a chicken full moult last Jan.

You can give her some extra treats as moulting can take allot out of a bird.