I can now join the catastrophic loss club! UK site, PR6, rock solid rankings, 400k pages indexed, fast loading - 90% loss of traffic since 2 days ago. Last time the site went AWOL was early February for a couple of days. Looking up what I used to rank for approx 20,00 medium to long phrases then all top listing just disappeared BUT results can now only be described as awful - in fact on one query results 11-20 contained sites from Poland, Germany, Czech, .info and not a single .com or .co.uk - when will Google admit that they really screwed up big time this time? As my site is a .com hosted on uk servers then it seems that I have now moved to the index for Mars. Time to move to the dark side methinks rather than stay in the Google lottery.
Google is not a lottery but a predictable game of hard work. Ranking do disappear but it could be either our own mistake of not following standards or it could the site linking to us has poured.It could be use of script which google doesn't permit or it could be stuffing. Reoptimize site, let google crawl again and wait to see results back. the loss of ranking has happend to me number of times but I have regained it with reoptimization and patience. Good Luck and keep your self in elite club.