Three of my sites were also hit by Panda and Penguin without explanation. Now trying to climb back out by fixing over-optimized pages and links. One site began coming up on its own as it was just Google Dance. It was good in a way as it forced me to unlearn my outdated SEO knowledge from circa 2001 and get educated on new things.
It has happend in various sites but most of them have bounced back. The key point here is to stick to the basic aim of promotion and do it the way they (at google) want.