An employee was requested by a client to string wire on top of an auditorium. He used a boom lift to access a beam and then exited the boom lift to walk on the beam. He then stepped down on a drop ceiling, lost balance, and fell. The drop ceiling caved in and the employee fell 20 feet onto bleachers sustaining injury to his abdominal area and lower left leg. The employee was wearing a harness, but it was not connected to an anchoring point at the time of the fall.