cabowabo - 7:18 pm on May 31, 2010 (gmt 0)
Your search keyword referral traffic ... which keywords users are finding your site. It was suggested here to compare:
Apr. '09, May '09, Apr. '10 and May '10. You are looking for massive drops ... but in your case, since it just happened recently, you could shorten your analysis to a two week window in each of the above months.