EDIT: Reworded & Cleaned Up to Ease Understanding

There has been a significant drop (90%) in traffic from Google to one of my well-established sites (6+ years) in the last week. Searches show that the top 3 wide keywords all dropped 4 spots. Searches for other keywords do not show ANY results in the first 10 to 25 pages of results, while previously one 1st or 2nd page at most.

Since there are 200 factors for ranking, the question really is:

What steps are necesary to figure out what caused such a drop?

There has been no major changes during this period or the last month on the site and certainly not in the homepage which has dropped rank. Over all the years of running this site and plenty of others, I have never experienced this. There are no duplicate content on my site and I have rigorously used canonical links for the last few years to ensure it is not misinterpreted as such.