You may try to put a thick soft metal pipe (aluminium or copper) over the drill bit (longer than the bit itself by ~10 mm) so it will touch the chuck's jaws, then GENTLY hit the other end of the pipe with a hammer several times, holding the pipe a bit askew, thus trying to disengage the jaws from the lock in their thread, and try to loosen the chuck after each series of hits. Just be careful with your fingers, and better hold the drill by the chuck while hammering it - so the hits won't go to the transmission unit (sorry for description - hope it's understandable, if not - ask for clafication from me)
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You may try to put a thick soft metal pipe (aluminium or copper) over the drill bit (longer than the bit itself by ~10 mm) so it will touch the chuck's jaws, then GENTLY hit the other end of the pipe with a hammer several times, holding the pipe a bit askew, thus trying to disengage the jaws from the lock in their thread, and try to loosen the chuck after each series of hits. Just be careful with your fingers, and better hold the drill by the chuck while hammering it - so the hits won't go to the transmission unit (sorry for description - hope it's understandable, if not - ask for clafication from me)