If your site are full of low quality content, can't easily accessed or slow navigation, full of advertising, spammy content and etc, for sure your site will be hit by Google Panda Update. I said this because that's what happened to my site. I've noted significant changes on my ranking and traffic after Google rolled out Panda Update. The mere fact that my site was ranking well for many years already.
But there was this so called course named Panda Breakthrough and it grabbed my attention. I just give it a try and followed the instructions on it. That would be including changing the navigation into a faster one, modifying how I wrote my content, adding more elements for the visitors and changing its design and more. After doing a couple of changes on my site, I'm ranking again. Thanks God I followed my instinct.