Help! Changing one buddypress admin setting has killed my site.

Hi

I've just been happily at work setting things up on my site when I wanted to see what happened if in the buddypress menu options I disabled the setting for allowing users to create and join groups (I want them to be able to participate in but not create groups).

I hit the radio button to select disable, clicked save settings and I got a white screen. Can't load up the main site, can't load up the admin pages. There's no ctrl-Z for something like this and the back-button is no help. All I get is a browser error:

HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfil the request.

I'm locked out -- have I lost everything?

Needless to say, this is a bit urgent...

Thanks!