did you still have sufficient pressure in the brakes, just no stopping happend?
did you use the brakes before on the same ride or was this the first moment on this perticular ride that you used brakes?
if you still had pressure in the brakes, and they were fine the day before, then i suspect some sort of lubricant most have gotten on your brake pads. they just don't fail on you from one day to the next, for no reason.
an external factor must be concidered...