DIAGNOSIS A metal shaving or debris is wedged between the bit and the collet balls, jamming them.
FIX Apply penetrating oil around the stuck bit. Let it soak. Grip the bit with pliers and twist while firmly pressing the release collar. Pull bit out.
IF THAT DIDN'T WORK The collet spring may have failed. You'll need to remove the anvil assembly to replace the collet spring and balls.
HEADS UP Never hammer the bit out. You will damage the anvil bearing and the impact mechanism.