The chances are that your brake light switch is jammed on. The switch is connected to a spring, which in turn, hooks onto your brake pedal. There is a picture of one of these units available here:
http://www.ebay.co.uk/itm/MOTORCYCLE-YAMAHA-TYPE-REAR-STOP-SWITCH-BRAKE-LIGHT-/310231989451 if you are unsure of what you are looking for. Two likely causes
1) The spring is over tensioned. There is an adjustable thread on the switch which allows you to move it closer to, or further away from the pdal, relaxing or tensing the spring. Release the tension on the spring by adjusting the switch unit so it is closer to the pedal (But test that when the brake is applied, the light DOES come on).
2) The unit is jammed as a result of age and or corrosion or dirt. Clean the unit, unclip the spring from the brake pedal and pull the centre pin of the switch in and out repeatedly to ease the movement. This may get it working again. If exercising the unit manually for 5 minutes doesn't fix the problem, you may need to replace it.