Hi

I have 5 pet chooks who are all about 2 yrs old (2 Orpingtons, 2 Faverolles, 1 silkie). They have always been good layers but have not been laying eggs for the past 4 months! It's winter in nz at the moment so I thought they might be too cold but all my friends hens are laying.. . Should I be concerned?
I am not sure whether you should be concerned or not.
It is common for chickens to stop laying in the fall as the days get short, which is also when they usually molt (lose their feathers and grow new ones.)
They typically start laying again as the days get longer in the spring.
Some breeds take a longer break than others, and the same hens may take a longer break when they are older as compared with when they were younger.
Providing artificial light (to make the days seem longer) can cause them to lay eggs during much of the winter. Sometimes just having them near a street light, or the windows of a house, or a busy street, can provide enough extra light that they lay when some other hens do not.
Those are all reasons that you might not need to be concerned.
But chickens may also stop laying eggs if they are fed the wrong food, or if they are sick, or if they have parasites, or sometimes if they are badly stressed. Any of those would be a reason for concern.
It is probably worth looking carefully at the chickens to see if they seem healthy, look for parasites like mites or lice, and check whether anything is likely to cause extra stress. It may be worth getting their droppings checked for parasites by a vet, or it may not be worth the cost & effort if they seem healthy otherwise.
"Wrong food" problems are usually when people feed something like straight corn, which is not enough to keep a hen healthy. But it can also be food that was stored wrong and got moldy, or food that was manufactured wrong because of some kind of error at the mill. If you think it might be the food, ask what your friends are feeding their hens (because that is obviously fine), then maybe try a bag of something different than what you have been using (to see if it helps).