I wouldn't do anything for 2-3 weeks. I know that this is detrimental to your business, but as far as I know google have become scumbags and now wait for people's reactions to their updates to see what they (the people) do. Apparently we are becoming too smart for google and google doesn't know how to deal with SEOs anymore, and now they play mind games with us.
You can take this as trolling on my part (although, I'm not trolling), but it's what some members of the forum have been saying after the last 2-3 google updates.
So, don't do anything for at least 2 weeks and if your rankings don't come back then it's time to act. And I would start with analyzing what google's last update (the one rolled out on the 22nd of July, so right before your site got affected) has targeted. Which, if I'm not wrong, it's targeted - yet again - product review sites.
If your site is a product review site then you've most likely be hit by this last update. If not, I don't know... you'll have to go through the last 2-3 updates, see what they've targeted, then compare your site against the changes and see what you can fix...
Either this, or you've build some bad links that have kicked in on the 31st of July and removing / disavowing them would probably fix your issues. I'd still wait 2-3 weeks before trying to fix what might not be broken, though...
