hi,
I take your question to be a black & decker drill ?. the method that has worked for me in the past is, close the chuck with the key tightly. Leaving the chuck key inserted, hold the machine facing away from you the key sticking out horizontally to the right give it a short sharp tap with a small hammer or mallet on the side of the key in a downward direction as the chuck has a right hand thread. Only on one of many occasions on a particulary stubborn chuck I damaged the key but the chuck still came loose. Good luck
john brunton (engineer retired ), January 2007