The PR is 0 because it has not been done seo or it does not have quality backlinks. If your site is coming on desired keywords than it is not penalized.
Pagerank is just a metric which compares your website to all other websites on the internet, the most linked-to pages get a pagerank 10 and the least linked to pages get a pagerank 0, keep in mind though that there is much more pagerank 0 pages than pagerank 10. If the toolbar is displaying grey for your pagerank or n/a it may mean that your website has been penalized.
PR=N/A just mean that google not yet give PR to the site (or saying new site) If the site get penalized, it will only go to 0, not N/A