The percentages don't help as much as knowing what the numbers are. To determine what is going on, it would take a complete site analysis, but here is what I can tell you to focus on.
1. Make sure your site has original, quality content. This is what both search engines and people coming to your site are looking for.
2. Make sure you develop quality inbound links to your site. Start by making sure you have done sufficient directory and search engine submissions.
The other question would be; has nothing at all changed with the site except for being listed at Google? It doesn't seem logical that your traffic would drop after being indexed in Google, so something else is likely going on.