DIAGNOSIS The keyless chuck has tightened beyond hand-tight force due to rotational inertia during use, a common issue with high-torque drills.
FIX Engage the forward/reverse selector to the reverse position. Lock the trigger in the low-speed setting. Firmly grasp the chuck sleeve with your full hand (not just fingers) and pull the trigger in short, sharp bursts. The jolting reverse torque will break the chuck loose. Then you can open it by hand.
IF THAT DIDN'T WORK Use a locking plier (Channel Lock) on the chuck sleeve. Pad the jaws with a rag to prevent marring. Grip tightly and tap the plier handle with a mallet in the counter-clockwise direction (as seen from the front) to break the initial seal.
HEADS UP Never use the chuck key to tighten a keyless chuck. It will over-torque it and damage the internal mechanism. The key is only for the optional auxiliary handle mount, not the chuck.