Sometimes, and probably more often, a drop in traffic is not a penalty. You're just ranking where you're supposed to rank. Sometimes and probably more often, a site may be temporarily re-ranked because the algo is still be fine-tuned. That last one usually sorts out in three to four months. The latter is referred to as collateral damage but I think that's incorrect because usually there was never a penalty in play.
It's not always easy to tell the difference between a penalty and being re-ranked because Google doesn't always communicate penalties via the toolbar, but thankfully there's GWT. In the old days the toolbar PageRank meter used to turn gray, usually accompanied by an absence in the SERPs. That was called being graybarred. That was bad. The other indication was the white bar. That was a little better.
Now that you have determined there's been a rank drop across all pages, perhaps it's time to check for technical issues that may be the cause of a ranking problem. Here is a
Dropped Site Checklist [webmasterworld.com] that has a list of some technical issues. Work through those first. One issue that is not covered in there is a poor implementation of https. I've come across a number of sites that can't be reached because of technical problems associated with a transition to https. Do a search for phrases on your site, look for duplicate content. That can be evidence of a server or software (CMS/Shopping Cart) misconfiguration. Retrace footsteps to any recent changes.
Then if none of those technical reasons are the cause then it's time to review past link building efforts, a review of the on-page content and look for irregularities.
