The chuck likely seized due to debris or lack of lubrication. You need to lock the spindle.
- Set the forward/reverse control button to the center OFF position.
- Insert the chuck key or a large Allen wrench into the gear ring at the base of the chuck.
- With the spindle locked, grip the chuck sleeve with your other hand and turn it counter-clockwise to loosen. You may need to tap the key gently with a mallet to break initial corrosion.
IF THAT DIDN'T WORK: The internal jaws may be jammed with metal chips. Try blowing compressed air into the chuck while rotating it. If it remains stuck, take it to an authorized service center. Do not disassemble the chuck yourself.
HEADS UP: To prevent this, always clean the chuck threads and blow out debris after use. Check for misalignment or binding of moving parts as part of regular maintenance.