Search Engine Journal has an informative post on Google penalties, a dressing down of sort that sites receive for not being regularly updated or for use of shady SEO practices. None of this has been accepted by Google ofcourse but these do provide a hint on whether your site has any issues that resulted in sudden fall of your site rank on Google.

An excerpt from Search Engine Journal

A member of the WebProWorld forums reported that there are 3 major kinds of penalties:

1. -30 penalty
2. -950 penalty
3. Index Exclusion

However, there has been a lot of debate on whether the drop of a website's rank by 30 or 950 spots in Google is really a penalty for adopting shady SEO methods or is it just because some of the major ranking elements that were holding the ranks of a website high just got devalued in the Google algorithm update tossing the ranks badly.

There are certain points to keep in mind to avoid these penalties. They have been discussed at SEOChat and Growler.