aakk9999 - 11:45 am on Aug 17, 2013 (gmt 0)
Have you tried to do the analysis to find out where the drop occured? Is it across all keywords / pages or a particular set of keywords / pages ? (I presume you have already checked that the traffic you lost was Google traffic and not direct / referrals.)
For example, if you go to WMT and select the period from 7th August to 15th of August and compare this with the same length period just before that - do you see any significant change in impressions or CTR ? Also note that the default chart there is "web", you may also want to change it to "image", "mobile" etc and see if anything gives more clues.