Take a look at your backlink profile as it might be that you've lost some good links or have some very bad ones pointing to your website so it might be a Google penalty
That is the dark side of SEO. One day you attack top positions and day after that your site drops a lot without any single explanation, or manual penalty. Just big G knows it
Google is going crazy right now.. i have reverted to biing to find what i need.. there is also massive news media campaigns targeting keywords which seem to be favoured.. intresting
Sometimes I wonder what the fuck is going on over at Google. Search results are shit these days, full of spam sites, hacked sites, Pintrest or whatever the hell it's called. And everyone doing the faq thing so it looks like shit and then you have the paid ads which take up loads of space so you're nearly pushed down to the second page somethings. Wasn't Google about serving customers the best and most helpful results?
All that probably matters for them now are big tech websites and Youtube, Wikipedia, etc.