The motion sensor unit shorted to due to rainwater infiltration, if its like the last half-dozen motion sensor units I opened up. Take the unit down, open up the sensor unit head, and if its got moisture inside, you know why it quit. If not moisture, then they sometimes do what all electronic devices eventually do, they fail due to 1 of the components reaching its failure - every sensor has an MTBF
(mean time between failure).

It may be a different problem. What I have learned from motion sensor lights that have been outside for years is that one day they stop operating the lights and yet the LED flashes to show that it is sensing.
Try this:
Get to your motion sensor head. Make note of where is faces and the angle it is on. Then rotate it so that all the switch adjustments are accessible. Operate each and every switch back and forth a few times and then put it back to their original positions. Then readjust the head to it's proper position and see what that does.
On my own Heath Zenith twin light bulb light fixtures w/ motion head, this got the units operating again for three out of three attempts.
NOTE: you can leave the power on while doing this adjustment. If you do, you'll find that the lights should come on as you switch through the "Test" setting and then when you put that switch back to a timed position, the lights will stay on for the timer count once even during daylight before going back off.
I believe that what happens is that years at the same switch positions they finally get dirty or weak or perhaps a small amount of moisture. Moving them back and forth creates a wiping action to restore their electrical connection properly.