Why has my outdoor security light stopped working?

Before calling an electrician, check these things: 1) The bulb — many security lights use replaceable lamps. Check and replace if needed. 2) The PIR (motion sensor) settings — most security lights have sensitivity and time adjustment dials. Try adjusting these. Some PIR sensors have a test mode. 3) The circuit MCB — go to the consumer unit and check the outdoor circuit MCB has not tripped. 4) Daylight sensor — many security lights have a light sensor that prevents them from activating in daylight. If yours is in a covered or shaded spot, it may think it is night and activate continuously, draining the bulb. If none of these help, moisture has likely entered the fitting and an electrician needs to check it.