Using automated queries from a script to calculate a google pagerank breaks google's terms of service, thus they're known to ban servers for it if the rate of requests is too high. You can test whether this is what's happening to you by creating this little test script (name it test.php if you like) in your main WSN directory and visit it in your web browser:
<?php require 'classes/pagerank.php';
$gpr = new pagerank();
$gpr->userAgent = $_SERVER["HTTP_USER_AGENT"];
$gpr->useCache = false;
$gpr->debug = true;
$rank = $gpr->GetPR("http://www.yahoo.com");
echo "The pagerank is $rank. Full raw data: {$gpr->rawdata}";
?>
$gpr = new pagerank();
$gpr->userAgent = $_SERVER["HTTP_USER_AGENT"];
$gpr->useCache = false;
$gpr->debug = true;
$rank = $gpr->GetPR("http://www.yahoo.com");
echo "The pagerank is $rank. Full raw data: {$gpr->rawdata}";
?>
If you see a message from google, you can appeal to google if you wish. If you've regenerated pageranks before, be sure not to do that again (let the system update them more slowly to avoid angering google).
If you see some other message, contact WSN support.