Some more information might help people to better respond to this situation.
1) Is your site still out of google.com?
2) How long has your site been live?
3) Was there any recent hosting issues?
4) Have you logged into Google webmaster central to see if Google had trouble crawling the site?
5) Not likely, but did you look to see if your site was hacked?
6) Have you recently made any big change to the site architecture?