I believe that you have two cruise control buttons on your steering wheel. One is for regular cruise control and the other one next to it with a car symbol on it is for adaptive cruise control.
Adaptive cruise control uses a radar sensor on the front of your car to ping the car in front of you to measure its distance, and then maintain a selectable distance from that car within a speed tolerance that you can set.
I think the icon on your instrument panel indicated that there was no vehicle within range of the radar sensor, so the system defaulted to regular cruise control.
You can test this conjecture by activating adaptive cruise control again when there is no vehicle in front of you.