I have a halogen security light in my back garden and in the last couple of nights been constantly on at night. Funnily enough, we had a power cut 2 days ago as well and it has happened since then. Before that though, it has been working fine.
I have tried to adjust the sensors on the light but doesnt seem to make a difference. I cant even see a switch or button to even switch it off till I get someone to have a look at it. I might take the bulb out tomorrow during the day so I dont burn my hands!
Anyway, any ideas as to why the light would just go constantly on?
On most motion sensitive lights in order to turn off the motion sensitive part and make the light stay on all of the time you flip the power switch on, then off, then on again quickly. This will cause the lights to stay on until you turn the switch off and on again. The power cut may have replicated this. Or it could be that the sensor is knackered. Go to the fuse board, turn off what ever fuse controls that light, count to ten put it back on, your light may go back to working normally.
I must say it's slightly concerning there is no isolation switch, suggests to me it's a diy job done badly. might be worth getting someone in to check it out.
Some PIR's can be set by turning power off and on to different functions the power cut may have changed the setting and you may need to switch power off/on so many times to return to configuration you require you need to read instructions for your unit.