Well, i am sorry to hear that for you. Just a quick question, did this drop on search results showed after or before the
pagerank update was completed ? If so, maybe your pagerank has gone down for your site and your concurrence's web site page ranking has went up. Also i would search back in your history and see for which keywords you were receiving more hits and if those are the same keywords you want to compete for, then start some
link building as text links with good
anchor text for your links or even better if you can build
link bait for your site. Carefully use your h1 tags, or place those tags for the important topics if you haven't done so yet.
We would need more informations to be able to mhmm, help you or suggest you what to do.