Before any real damage is done (hopefully):Despite common thinking even drill chucks work better if properly lubricated - a dry chuck seizes up quickly...There are two different kinds of blocked chucks.a) the nut/head closing the chukc is stuck.b) the nut is loose but the three prongs still won't budge.For a it is best to dismantle the drill so you can use gripping pliers or similar instead of this tiny pin locking the chuck - this pin tends to break out with great froce applied.For b the best way I found is to remove the head or at leastget it loose a little bit, then use a rin, pipe or similar that fits onto the three prongs and give it a nice tap - this will free the prongs.
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I agree with iceng, that if possible heat the outside of the chuck gently then you should be able to loosen the chuck.
Good luck, I hope you succeed
If the chuck is loosened but the drill bit is still stuck in it, try pushing the drill bit in and then remove it. I've ran in to that problem on keyed chucks before, so it might possibly happen on keyless as well.
if you are not strong enough you may need to use a tool, like a plier called channellock, to undo the chuck... Or you could try heating the chuck which expands metal parts making them easier to undo...