Analyze your backlinks and index status. If you found any lag/problem in them, probably your website might be penalize by Google. Else make use of Google Webmaster Central help forum.
It may be a normal fluctuation so stay patient as it will be normal soon.
If not then you need to check whether your site is partially indexed or fully deindexed using the site: command. If its deindexed then it can be a SERP drop or a penalty.
Check for stale content on your website, links from bad neighbourhoods, malware or your site may be hacked.
Here’s Matt Cutts explaining the possible reasons for drop in ranking, http://www.youtube.com/watch?v=gx04qNrCbnk
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