Sempervivum are tough little succulent plants. They can take most punishment that their owners inflict on them. These little plants punch through neglect like it's nothing. Drought doesn't phase them and cold just makes them more colorful. Despite these super powers, they have a weakness. Their kryptonite is cold, wet feet.
Hens and Chicks do not like to have their roots sitting in cold water. If the plant is dry and cold, it will be okay. If it's summer time and the plant is in a puddle then it can pull through (I've had a semp sit in a pot with no drainage and kept a visible layer of water on the surface for over a month before and the plant survived). Mix those two things, cold and water logged roots, and it's the perfect recipe for root rot.
Root rot is just what is sounds like. The roots are surrounded by water so they can't breath and they start to decay. Root root is deadly to plants and there isn't a good way to treat it.
The one good thing about root rot on your sempervivum is that it does let you know that you need to provide better draining soil or water less frequently. The photo below shows just what a rotting plant looks like.