BuddyPress.org

Have you upgraded your wordpress Mu to wpmu 2.9.2 or you are using the old version of wpmu below 2.9.

For now, you can simply rename wp-content/buddypress and you will be able to access the wp-admin. Let us know about your current configuration in details as mentioned here
https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/when-asking-for-support