When you get big changes it is often worth checking again this page here on seomoz: http://www.seomoz.org/google-algorithm-change
That shows a calendar of big updates at Google, although no need to check it for that date: You've probably been hit by the penguin update.
You'll find LOADS written about penguin around the web. However the 2 minute version is that it is an update that looks at poor quality link profiles. Most sites hit badly had a substantial proportion of unnatural looking links: largely those from poor quality sites and/or over optimised anchor text.
Most sites didn't get notifications in webmaster tools for this.
I hope that this helps. There conflicting suggestions as to the best approach to recover. However the general principal is to ensure that a higher proportion of the links pointing to your site are/appear to be quality, natural references.
