Hi there,
I have the same old problem as always, but slightly different case. I had my WPMU running, but then I upgraded my hosting, they changed server and since then I get the fatal error.
Everything was configured fine, running, but not now with same files in different server, what leads me to thing is something related to Apache?
Any ideas?
Have you checked the database(s)? Are they still there? Do the tables look alright? They're not corrupt?
Gotta admit I would be sitting on your host right now. They've done something to break your site. At the very least get them to tell you specifically what upgrading they've done. Like php4 -> php5 maybe? Can't really give you any ideas unless we know what specifically they've done to your site.
everything looks fine, but for any reason I get the NO WPMU Site defined. I was moved to a different server so it might be a apache thing?