I am the Principal of BCCO LLC, an Internet Marketing company. I specialize in SEO, SEO audits, search engine penalty recovery and more.
This is a very open ended question. I would need to be able to see the website in question and gain access to google webmaster tools and analytics.
I can say, if you fell entirely out of Google's results, it is a major error or you were caught doing some aggressive spamming.
If it was a minor drop, it may be normal fluctuations.