Hi; For one of my website, I observed drop in ranking after mid of Sep, 2015. Although there is not any Google Algo update then why this happened to my website? Any suggestion to move back and how can I restore or improve my ranking?
Rankings drop all the time without updates, it's normal. You just need to build more backlinks to get it more stable.
Go through with Google Webmaster, and check "link to site" section, and see how many repeated and useful links are there. also try to re optimized your website.
Algorithm update is not only the reason why the other websites dropped their rankings and the possible valid cause was the other competitors sites are over performing your site which means they have better engagements and great user experience.