A loader operator thought EE had a leak on a 980C loader. EE parked the loader and climbed into the front right fender well to inspect for the leak. As EE was exiting the area, EE dislodged EE's hard hat on the fender, and it fell to the ground. EE bent over and picked up EE's hard hat, raised back up; and struck EE's unprotected head on the fender.