Senseless things have no meaning — you can't easily explain why they happen. Senseless people, on the other hand, are simply unconscious.
This adjective often describes something terrible or violent, like a senseless accident or the senseless destruction of a city. It can also mean "done without thinking," or "lacking understanding," so a senseless student will give the teacher a blank look when their name is called, and a hiking trip across the Rockies in the winter would be senseless too. The "unconscious" meaning is the oldest, dating from the 1550s.