A cement crew had finished a surface cement job and was preparing to rig down the cement head and equipment. An employee was preparing to remove the cementing head and manifold from the casing. The drilling rig crew loosened the chain boomer and lifted the casing to remove the slips. When the rig operator lifted the casing, the hook on the chain boomer broke, striking the employee in the hard hat and knocking him to the rig floor. He hit his head on the floor, losing consciousness and suffering a head laceration.