Please help me about this message, Google Webmaster Tools notice of detected unnatural links: due to this message I observed my website's search engine ranking had been very down day by day. So what will be do please send me reply thanks in advance.
Unfortunately google do not say which links they are, and that is what makes it difficult. But you must go through them all, and try to remove those which are not links you would want to your site. Keep a record of what you do, so that you can submit for reconsideration and show evidence of your efforts to remove the links.
First, you would have to identify where those links are and have them removed. It all depends on where they are. For instance if those links were blog commenting links, you would to contact the blog owner and have them removed which may be hard. If they were links that you create trough your own account to a site, you can just logging in and have them removed. The question I'd like to ask how do you know that those links have been detected? Have you been building them too fast?