I have an external halogen lamp on a PIR sensor that used to activate on movement. It is now permanently on and the switches that control sensitivity do nat affect it. How can I reset it?

You can try to reset the PIR sensor by disconnecting the power for a minute.
That failing to have any effect, the sensor requires replacement.
Note : make sure your sensor has the correct capacity for switching the halogen lamp.

To add to sarnian's, if rated for 150 Watts, then 2 100 watt bulbs may destroy motion.
Many times when they fail, they fail in the on condition.