Changing your site from table-based layout to pure CSS is a MAJOR change.
Google seems not to appreciate major site-wide changes as they're usually indicative of spam. If your text to code ratio has changed significantly, Google might be worried that your domain has been overtaken by yet another spammy advertiser.
Most often, they'll penalize these sites to discourage this sort of thing, so this might be what you're experiencing. Resubmit your site map and your rankings should pick up after a few weeks.
My site went AWOL for six weeks until I resubmitted my sitemap and sent an email requesting reinclusion in the Google index, so that's something else you can do.
Give it some time. It takes a few weeks to be reincluded, so use that time to fine tune your
SEO efforts and get everything ship-shape.
Here's a handy article that might be of some help:
http://www.mattcutts.com/blog/reincl...request-howto/