3 options I can immediately think of...fuse, switch and cruise-control module. Begin with the two easy ones, fuse and switch. Find the fuse and make sure it's not blown. Then use a multimeter to check the switch for continuity. Using a multimeter on the switches "inlets"/tabs/wires or what ever you can tap into, measure resistance with the switch on and off. I believe with the switch unpressed you should read open, "inf", "overload" or something to that effect. With the switch pressed you should read a very small resistance. If there is no change when you press the switch, the switch is dead and needs to be replaced. If the switch reads ok, you may unfortunately be in need of a new cruise control module...though maybe other contributors have further advice on this one?