An employee was releasing cargo chains tied around an excavator in a cargo hold on level 1 of the vessel. During that time, some of the latch hooks on the chains became wedged and would not budge. A coworker used a pry bar to help release the chains. The coworker pulled on the pry bar, which slipped from the coworker's grasp and flew towards the injured employee. The pry bar struck the injured employee in the head, requiring hospitalization.