Brake arm bolt fell out, allowing the caliper to rotate and lift, allowing the pads to fall and be ground by the disc wave edge. Few hard applications of the brake have cut through to the piston. Looks an easy fix if you could machine down the end of the brake piston. Even bung something in the end of it, like a lump of wood, squared off or a 1/2" drive socket. New pads and a bolt = back in the game.