did you give them to nearly dark to come in? sometimes my chickens for whatever reason decide some nights they want to stay out a little later than usual...
Do you have any large trees around? It is quite possible they felt safer high up in a tree, and roosted there. I looked for 20 mins for one of my chickens when it was just 11 weeks old or so before i found it roosted above my head in the rafters of my old coop.
They will be back in the morning, long as they were able to avoid any predators.
Do you only put food and water in their coop? no option of food or water outside the coop? you could also try luring them into the coop at night with treats.. meal worms, or even table scraps like bread or left over veggie scraps. When I have an issue with my older girls staying out too late i can always coax them in the coop with sunflower seeds or other treats. They know whenever they hear "hear chick chick" they are getting treats, an boy do they run for me when they hear me call. You can easily treat your birds this too. just anytime you're giving them treats always call them first, even if they're right in front of you already, that way they associate being called with treats, and they will learn to always come when called.. expecting treats.
