I had top search results on certain keywords with some of my articles. but google started to ignore them now and it completely ignores all pages with this keyword. I lost top search position and other sites that copied my article and even gave referral link back to the article are on top results. my site is listed in google, crawl able, and with no infection report on google webmaster tools. What caused this and how can I fix it?
Sounds like it was the Penguin Update. Google changed its algorithm fairly recently in what is known as the Penguin Update (they seem to like monochrome animals!).
In it the seemed to drop a lot of sites that used outdated ranking techniques, especially those sites that had lots of links coming in from unrelated websites. But to be honest right now nobody knows exactly what has happened, people running multiple websites with the same SEO strategies are seeing some sites hit and not others.
Here is an article that goes into things in a little more depth: http://searchenginewatch.com/article/2172565/Hit-by-Google-Penguin-An-In-House-SEO-Crisis-Management-Plan