I am (was) a happy user of Multisite WP. Installed through Fantastico, enabled multisite, then eventually upgraded until 3.1.
Fantastico reported WP was out of date and proposed to update. I clicked on OK (id it on other occasions, no problem - besides, the hoster requires all Fantastico update warnings to be taken care of).
Disaster: users complain their blogs is gone. I can log on to the blog, but is single site. DB has been restored, users are therz, but I can't see the sites.
.htaccess for multisite still in place.
I added "define( 'WP_ALLOW_MULTISITE', true );"
And now WP detects the existing network, but I don't know how to access it - add it to the site again etc.