Go to Google Webmaster Tools and see if there are any messages for you there: https://www.google.c...tools/messages?hl=en
Sometimes if they take away your page rank they have also removed pages from their index, and they may tell you why, there.
One of the things not mentioned by mahesh2k is that they also take away PR if they think you are doing paid posting.
Also be careful about what directories you are submitting your blog to. If there are too many splogs in the directory, you can get punished for being listed along with them (birds of a feather flock together?)
And be very careful about quoting in your posts. Too much quoting can get you slapped for duplicate content and labeled as a splog. Make sure your own original content outweighs the quoting by at least 3:1, which means you should be writing 3 times more than you quote. Quote 100 words and you need to add 300 of your own to balance it.
google does update pagerank occasionally but.. 0 seems weird, like a bug.
Not a bug. I was hit hard on my ebook site last year and went from PR4 to 0. Just got myself up to 2 again. Reason? Having the "wrong" TLD. It seems that nearly everyone that had a .tk domain name associated with their site was slapped at the same time and reduced to 0.
It is Google's position that only spammers, splogs, and malware sites use .tk domain names and if you use one you must be a bad site. It doesn't matter how many inbound links you have, or what the quality, reputation, & PR of the sites linking to you is, or if your site was on the front page of digg... if you have a .tk you are crap in their eyes.
I am glad it came when it did though. I was just about to buy the domain I was using and go from free to paid status. I would hate to have paid for 9 years of that domain name and then get hit.