Eek, I'm sorry that happened to you. I'm going to repeat what the above posters asked, which is whether or not you recently made any changes AT ALL? Even things you would not have associated with your rankings. A dramatic website makeover? Changes to the homepage? Dramatic change to page content? Any new or different SEO or marketing efforts? Can you pinpoint the change having occurred at the same time as ANYTHING that you recently did?
In my own experiences, such an old domain would usually have a really stable SERP for your targeted keywords and it should be fully trusted by Google and other engines. I would like to recommend you to have webmaster accounts with all top 3 search engines and find out if could find some clues via webmaster centers.