When we say that the garage door cable snapped, we actually mean that the cable broke. The bad thing with snapping cables is that they have a great amount of tension which might hurt people standing by at the time of the snapping. It's not hard to notice when cables become old. They usually fray eventually and this would be the perfect time to replace them before they break. Their job is to keep the weight of the door and lift it along the springs. So, as parts of the extension or torsion spring system, they can be very dangerous.
When cables snap, they either end up on the floor or left hanging from the pulley or the cable drum. They don't usually break at both sides together but it's not unlikely. So, the overhead door might be sagging to one side if only one cable snaps. Its replacement must take place immediately for convenience, security and safety reasons and it's best to be performed by a professional technician.