I've actually had this happen with a new site. In my case, the site didn't disappear from Google's index entirely--it just kept bouncing around from page 1 search results to like page 5. I was able to eventually find my listings by typing exact titles in the search results or just the domain. Did you type "site:
www.YOUR_DOMAIN_NAME.com" into the Google search bar? You coudl at least verify thsi way that your site isn't completely gone.
I would also fix those errors found in webmaster tools immediately and make sure you have a quality site linking to yours.