Copied from: http://buddypress.org/forums/topic/cant-login-or-access-admin-panel
I have WordPress MU version 2.8.2 installed. I have BuddyPress 1.0.3 installed. I have bbpress 1.0.1 installed. I had been running 2.7.1 with 1.0.3 for a few weeks till yesterday morning. Something went wrong and i wasn't able to login to the main site or any of the domain mapped blogs. Everytime I tried, it would accept the username and password and then freeze at a white screen. Trying to access /wp-admin/ on any of the domains or the original site caused the same white screen. I realized there was a problem and deleted all of the files (after making a backup). I then used svn to install 2.8.1 and 1.0.3 in the same directory that the other one was installed in and ran the upgrade scripts on the same database. All was well. I re-added my plugins and all was well till a few hours later. At that point, I was unable to login again and since then the whitescreens have continued.
I have received emails from users who have been unable to login as well. I updated svn to the latest trunk and called the update script. There were no changes. There is nothing that appears relevant in apache httpd logs.
WPMU is installed at http://wpmu.zyrot.com/ buddypress is running on the default blog #1 http://wpmu.zyrot.com/ and bbpress is installed on http://wpmu.zyrot.com/forums/
Thanks in advance.