Hello Tristan,
There is a disease in these sorts of breeds called EDS (Egg Drop Syndrome), although I would be surprised if it affected 20 chickens all at once, even though it is viral. Do you know if they were vaccinated as chicks against this if you didn't raise them yourself or bought them at POL.
I had one of these birds who never laid a single egg about a year and a half ago. I had bought her as a chick and she had not been vaccined against this. she was constantly bullied by the others, and in the end I couldn't afford to keep her. I took her back to where I bought her from and they gave me a chick for free as a refund along with a couple of others.
Check for mite infestations as well because the stress of this can stop laying. Its definitely not the age of the birds as you said, and the breed normally continues laying through the winter. Do check for sure the eggs are not being stolen as Chris says. Its not uncommon unfortunately
There is a whole host of reasons why they could have stopped laying. Bad weather, frights as you've said. I would think the most likely reason at this time of year would an infestation of worms, or some other health problem. Are you fairly strict on the biosecurity etc, especially at this time of year with extra mud and health risks. How often do you clean them out and what sort of litter system do yo use e.g. deep litter, and the bedding type.
If they are definitely completely healthy, then its probably just natural. They deserve a break and I wouldn't worry too much. If they haven't started laying again by the time it warms up again in spring I would definitely be worried though given the breed and time of year.