Check whether you are using any duplicate content or fast backlinks. Go slow on building them as google will penalize if we overdo this backlink process
You should also check for any images that may be corrupt. This can be a problem with some banner ads. You'll have to use webmaster tools to see if there was any problems that the spider has. Also , check your analytics to compare what keywords you were getting traffic from and if there was a change in any specific keyword or keyword phrase.
Try adding the following meta tag in the website or in the robots.txt <meta content='all, index, follow' name='googlebot'/> <meta content='all, index, follow' name='robots'/> Resubmit your sitemaps. Use a free website submission tool ( freewebsubmission [dat] com) and submit your website to all major search engines. Also check your keywords. I recently changed my keywords in meta tag, which brought more traffic from both Google & Yahoo.
I have heard of this and they insert code into many of wordpress php and index pages. It affected many many sites.
What you need to do is promote Yahoo or Bing any time you can, every site you visit. Google is turning into a joke and people have been noticing it lately. Visit webmasterworld. Google needs a strong competitor.
I also suggest you to target your competition who are listed on the first page now. How many backlinks they have for their site and with which anchor text and if they are doing better then you, start getting links from the moment. Another thing can be checked and i.e. if any site who is linking to you is down now.
First determine what keyword was giving you the most traffic previously. See if that is effected as maybe some competitors have beaten you to it. Second, stay put and see if your traffic returns after a few days as it probably was just the traffic in your niche that is declining.