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: Assuming that your brake pads are fine, the indicator being constantly on may be caused by a faulty sensor, or it may be caused because the sensor has become unplugged.

: If the sensor is unplugged, simply plug it back in. If the sensor is faulty, replace the sensor. Trouble here is that I have never been able to remove old sensors from existing brake pad. So you may have to replace the pads.

: Sometimes heat build up from the brake melts the sensor wire, so this causes a fault. In this case, what you can do is reattach the wire with some speaker wire.

: Hope this helps.
I've removed and replaced the sensors lots of times when changing pads. I just gently pry the sensor out of the old pad, clean it with brake cleaning spray, and install it on the new pad. If you take care they will be reusable many times.