Forget my question : I was stuck cause 3 hours ago I made the wrong answer (I hadn't realized it was the black gate quest compulsory for the victory quest so I continued as if it wasn't important and it's only 3 hours later, after having explored the whole map that I realized my misstake) so I took an old save and voila ! Problem solved !

Actually, I think a 'no' response should not be possible.  I think all quests (at this stage) should replace the 'no' with "I have no time for this,' or 'perhaps later, not now.' The game is best with many options, and shutting one off, irrevocably, seems poor form... oh, do I hear a ticking crocodile?