Your site ranks for your desired keyword phrases in Yahoo and Bing, but is nowhere to be found in Google. Why doesn't Google like your site? Or maybe your site was ranking well in Google, but has suddenly plummeted.
Whether you are an SEO superstar, or just trying to get your site more exposure, it is important to realize that rankings are not static and will fluctuate. It's a common mistake to get comfortable once you rank for your various keyword phrases. Once you achieve your ranking goals, remember you won't stay there forever without hard work, and even then, you will experience ranking fluctuations.
First, don't worry that your page was penalized unless you have done something deceptive. If you aren't doing anything "fishy" with the search engines, you won't have to worry about being penalized. Most likely, if your page rank suddenly drops, it's due to a major algorithm shift. Search engines are relentlessly fine-tuning algorithms, and new sites are continuously created. As a result, ranking fluctuations are normal.
Since this is the case, it is absolutely critical to optimize your website for many different related phrases. This will help when some phrases suddenly drop, as there will be others that perform well. Never get stuck on one or two phrases. It's awesome to rank well for some of the more coveted phrases. However, if they are that significant to you, you may want to also consider running PPC campaigns targeting your favorite keywords.
I recommend changing your focus from rankings to targeted traffic and conversion rates. Focus on making your site efficient at converting traffic into sales. With a high conversion rate and a well managed back-up PPC campaign, you won't even notice a blip in your traffic or sales.