mid-May, eh? That's an obvious sign of Penguin penalties, just as EGOL is saying. Sounds to me like you built some icky links somewhere along the way and didn't spread your anchor text out enough. DA isn't everything and Google doesn't technically pay any attention to those numbers... keep that in mind.
The best thing you can do is build content. I feel like a stuck record, but it's true. The better the content, the more links you will build organically which will then come from relevant sites. Look into backlink profiles and do some research on what they should look like then compare that to yours. Use OSE and ahrefs.com to do this.. check out nike.com, forbes.com, and especially your competitor's sites that are outranking you as well as whatever big name brands you can think of. I bet you will see a lot of keyword-heavy anchor text in yours and not so much in theirs..
And as always, check your webmaster tools account to ensure you haven't received a manual penalty notification from Google's Webspam team. If you have... good luck.
Actually, good luck either way! These forums will help you throughout the process. And as I always say, providing actual URLs will help folk here provide you with less speculative answers and more solid fact.
Onwards through the SEM jungles my friend!