goodroi - 2:07 pm on Nov 6, 2012 (gmt 0)
I would double check the robots.txt and then triple check it. A single typo mistake in robots.txt can cause huge problems. Also check to see if you are using the meta robots tag on the page.
Then check your log file to see how Googlebot has been behaving on your website and for how long. This can help identify the day that the issue was introduced to your website.