I've had this happen to my very small flock too. I don't have a lot of experience, I've only had my birds for a few years, but I can pass on what I've learned.
There are lots of reasons why chickens stop laying. Mine stop when they're molting, when they're broody, and when the nights are long, in the winter.
About a year ago, a couple of my chickens stopped laying and I contacted the cooperative extension, which told me that they were probably reacting to a series of stresses that occurred one after the other: change in weather, change in food, storms. One of the hens started up again but the other didn't and the cooperative extension told me she may be finished her laying years. That usually doesn't happen suddenly, though, they trail off.
It turned out that the hen that stopped was suffering from avian infectious hepatitis. The first signs apparently is that they stop laying and become withdrawn (go off by themselves, act "depressed"). I did notice that she seemed less aggressive and less energetic and curious. This is a contagious disease but some chickens are immune and some are asymptomatic. It is treated with terramycin.
I have no idea whether it is likely that this is your problem but one thing I have learned is to take all changes seriously and keep a close eye on them.
