check the switch behind your right foot peg. make sure the spring did not pop off. also there is a barrell type adjuster on the switch, make sure there is enough tension on the spring to activate the switch when you push the brake pedal.
Here is a diagram. You don't have to unbolt the rearset to adjust it. Like he said above. Check the spring and the adjusting nuts. The spring stretches over time which prevents it from activating the switch. Also check the wires to the switch. If that doesn't help then it may be your switch is faulty.