fionashanahan, If really stuck, give a touch of penetrate oil down the jaws of the chuck, put the drill in reverse, firmly grip the chuck with gloved hand and gently squeeze the trigger up the point that you cannot hold on. If it still doesn't budge and you're not able to hold the chuck against the force of the drill. Get 2 strap wrenches(they will not damage the chuck) one to hold the chuck and one to hold the collar, may need an assistant. With the strap wrenches both in position, adjust the slack out of them firmly hold the collar with the one wrench and turn the other in counter clockwise direction to break chuck free. Posted a couple video links below that may help. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.
https://www.youtube.com/watch?v=iPQTwLQsHFY
https://www.youtube.com/watch?v=R0hQ9nnX9KU