goodroi - 6:28 pm on Jul 26, 2009 (gmt 0)

If you have been using that sitemap for months without any issues I would suspect a temporary server glitch. The server your website is hosted on may have been overwhelmed when Google attempted to visit.

Simply having a sitemap does not guarantee Google will index your pages. They try to only index pages with unique content. Also if you have no backlinks or very minimal backlinks that will further decrease the chance Google will index your pages.

A sitemap will help improve the odds of Google indexing your content. It is much more important to have unique content with significant backlinks.