An employee fell from ladder to the ground and sustained multiple injuries. He was climbing from a stairwell platform to access the roof hatch to assess the HVAC unit. The distance from the platform to the roof is 12 feet. As he was climbing through the roof hatch the ladder slid from its position and the employee fell backward, head first onto the ladder that had become wedged between the stairwell walls. His left leg/boot became caught in one of the ladder rungs and he was suspended from the ladder. He managed to free his left foot before landing on the stairwell platform. He was found on the floor, lying on his back with a portion of the ladder on top of him and experienced pain in his left shoulder and left arm.