why wouldn't you be able to find out what happened? Do a comparison using the new date range where everything plummeted against a previous date where things were humming along. Go to pages and sort by click/impression differences to see the largest negative numbers. Those are the pages that dropped. Hone in on those and click queries to figure out what queries are no longer driving impressions/clicks. 4 minute analysis to find out where you lost. Fixing it may not be an option, but maybe it is.