If you think about Google and what they do you must understand that they are indexing / crawling million’s of pages daily and when that data then forms the database that is the search engine Google once in awhile they might “loose” a site. Such errors correct them self’s normally within couple of day’s or a week and your site is back to its normal self.
Let’s say that you site is a large informative content site that best to your knowledge has not been the victim of any spam techniques. I would how ever recommend that you do track all past changes done do the site for the last 3 – 4 months that might be change of CMS, hosting, IP, and some programming ad-once and so on. Don’t reverse to any doggy panic actions, just stay calm and backtrack any changes done.
One of the things you can do is to register to Google Webmaster tools, upload your sites XML site map and look for any indexing error Google might be encountering.
Review your analytics solution, when did your site drop? I also use Indextools to alert me if certain keywords fall under a given minimum as search engine referrer.
As you register to the Google Webmaster tools I would also review their webmaster guidelines to see if you have done any changes that are against their rules.
If this does not help get a SEO expert to look at the problem for you.
When thinks are up and running again and back to normal, be careful when you do SEO changes, you only have one site and if it goes bad you will have to start from scratch.