Has your website been hit by penguin or panda?
I’ve see businesses decimated because of Google’s famous pets. It’s sad, but there it is possible to fix all that one way or the other.
If you have a lot of duplicate content or thin content then most likely you were hit by panda and most of the time this would be more easy to fix than a slap from penguin. If you have been practicing spammy link building then the culprit is penguin.
Looking through all those backlinks take a lot of time if you plan to copy and paste.