A page of my site is removed from search results! Until 2 or 3 days ago this web page was the second link in google search results while searching for specific keywords.

But last day I realized that it is not shown anymore! I made lots of search for finding the reason and now I am supposed either I am influenced by google penalty or it is because of my site duplicate content! I checked the google webmaster tools for a manual penalty but nth was there. Hence, I guess it can be an algorithmic penalty by panda perhaps. To be sure, I checked the last update date for google algorithms (the ones published officially) but the last update was a month ago! So I am not sure if it was really a penalty or not (Confused). How can I be sure?

Unfortunately I had hided some keywords in this page for increasing ranking and this is exactly the page disappearing.

On the other hand, my site suffers from "duplicate content problem"(because of the pages I have removed but are still indexed!) that I am trying to handle. Can it be the reason ?

Please tell me what should I do if it is an algorithmic penalty by google? Can I simply remove the hided texts and be hopeful google will reconsider me while the next crawl? Or I should make a new menu with a different name and copy the information in the previous webpage inside with some changes?