I'm having the same issue, which just started happening today strangly. I had been using it on a multisite network with no problems for a while. Then I started getting the following error in my error logs a little while ago:
PHP Fatal error: Class 'PageNavi_Core' not found in /path/to/wp-content/plugins/wp-pagenavi/wp-pagenavi.php on line 54
That was causing a white screen everywhere. So I manually decativated it by renaming the wp-pagnavi directory. After that I was able to get into the WP Admin since that deactivates it. Then I tried to reactivate it and the message in the WP Admin just says it triggered a fatal error.
I'm using version 2.82 with WP 3.3.2 and PHP 5.2.6.