Hi Many possibilities abound but here are a few'
1 sensor is a separately powered piece of kit in its own box usually dumped on top of the cabinets ,this has a relay (which you hear clicking) which switches the downlights on.
2 You have a 'sensorlight ' system. A custom unit which contains everything including the downlight feeds.
3 You have a sensor and power supply which then drives a separate relay unit which again controls the downlights.
Best bet is to investigate the wall unit tops for metal boxes (or if the job has been done correctly) a control unit mounted in a cupboard or on the wall above the units.
Look for a switched fused spur (similar to an immersion heater type switch) connected to the unit.
Any metal boxes should be marked and may have a low voltage fuse for the downlights that has blown.
Or as (unfortunately) in most cases all the downlights are chocblocked to a free standing transformer ,look for a loose wire.
It goes without saying turn of the mains before checking any wiring.
HTH CJ