I'm really sorry to hear about your drop.
Did the drop coincide with any other changes? (Site Redesign, etc...)
Do you have a Google Webmaster Tools account? If so have you checked for any recent messages and/or new errors on your site? https://www.google.com/webmasters/tools/
Did you notice a similar drop in your google and bing rankings or just google?
Some likely reasons your site might drop:
Change in the Search Engine Ranking Algorithm: http://mozcast.com/ is a great tool to see if your drop coincides with other search engine changes.
Malware or Hacking, Penalty Server Problem:, Webmaster tools is the best place to start looking.
Loss of PageRank/Link Popularity, or Canonicalization/Duplicate Content Problems: Use your SEO Campaign to see if you have any duplcate content issues, or changes in backlink profile.