Re: Can't Login or Access Admin Panel
Theme files can also cause this issue–although usually you don’t see it WPMU’s backend.
Are you able to login as the Site Admin, or does that also bring up the WSOD (white screen of death)?
I suggest switching to the default BP themes and see if you can login. If you can, then it has something to do with your custom theme–some recent change in the template coding, or possibly a missing custom theme file.
If need be, you can manually change the theme selections in wp_1_options (the options table for the main site blog user) and also in wp_sitemeta. If you go this route, make sure that you first completely backup your database.
Be careful with this procedure as it could lead to more problems. Manually editing data in the backend DB tables is never something you should consider unless you know what you are doing.