From a programming/development perspective, what would cause a site to practically disappear from the search engines after a site re-design goes live?

I have no current information on much - except that the site dropped from being on page 1 for many main keyword phrases to the deep dark "bowels" of the SEs...

I probably can't answer questions as I am new to the project, brought in only "after the fact" for their SEO. And it seems that it's been a couple months since their decline to the "depths".

So far, I can't seem to "see" anything too deeply amiss from an SEO perspective, so I thought I'd tap the techie world for some speculation and thoughts...

I do realize that it's hard to speculate w/o seeing the site's code, but was wondering if there were any things off the top of your heads that might come to you that I hadn't thought of yet...

Any takers?

Thank you in advance! :o)