The first thing to do is wait a little while. Sometimes ranking losses from G Updates are not permanent. So before changing anything, give it some time to settle (a week or two is sufficient).

The typical SEO strategy is to first identify what the G Update was for (if possible), then examine the site's backlink profile and take action accordingly.

In addition, you'll want to examine the backlink structure of the sites that link to your site.

If those actions do not work, and you must be ranking in G, then this is really troublesome. I'd recommend you contact a SEO professional immediately for an evaluation of the site.

Cheers --
Nick
http://nickpassig.com

##