I am not sure I understand your situation. When you say the "link of the same keyword is still their" do you mean your backlinks are unchanged or do you mean something else?
Did you make any changes before the drop in traffic? I would look at analytics to find which keywords & urls have lost traffic. Then I would focus on them looking at for any change in backlinks, page content etc. Then I would start looking at possible penalties like over optimization, bad backlinks etc.