Search Engine Journal has a good post on reasons that your blog might be banned (or penalized) by Google. So if your blog has disappeared from their index you might want to consider one of these seven issues:
1) Duplicate Content – where multiple pages have the same content.
2) Cloaking – where pages are created purely for Search Engines – ie where readers get one version and SEs another
3) Hidden text or hidden links – links that are not seen by actual readers that are there purely for SE spiders
4) Keyword stuffing – putting too many of your keywords in your page (either in meta tags, invisible text, in image tags etc).
5) Linking to bad neighborhoods – linking to pages that use Spam techniques – or link exchange programs.
6) Buying links for Search engine ranking – hard for Google to detect but if they catch it penalties occur.
7) Machine Generated Web sites – spamy type sites created by automated systems – hundreds and thousands of pages at a time (usually only slightly different from one another).
Read more at Banned from Google and Wondering Why?