Need Help!!!! My Index Bounced Back From No. 1 25th Nov
 

Hi All
One of the index of websites was on first page for its niche keyword, but in nov last week, it was bounced to the 9th or 10th pages. Also for the secondary Keyowrd, it was bounced back. Other pages though staying on the same for all of their keywords. Any help? Should I wait sometime or instantly do link building for home page?
Also suggest some best strategy to recover this during dec,jan.