Check Your Google Webmasters Tools After Your Pagerank Down

Many indonesian new bloggers asked me why their blog pagerank down from 3 or 2 to n/a (not available). I just tell them that my pagerank also down, and then I check my google webmasters tools. First, I thought that my pagerank down because of a malware that had infected my blog so I report Google to fix it. But, Google didn't response my requirement.
So, I saw into external link site info in google webmasters tools, and I check some dofollow pages where I had commented into them. I was surprised because in reality many pages with high rank became n/a (not available), and I resume that probably my blog pagerank down because many external links became not available and they were banned by google.
I am now must comment to other pages with high pagerank to increase my pagerank again.