My site has diasappeared after installing WPMU plugin.

I downloaded WPMU last night... started to activate plugins... went to bed. This morning i got up and went to my website, and I NO LONGER HAVE ONE! All I get when i point to my url is this message referring to WPMU's Admin Ads Plugin "The Admin Ads plugin is only compatible with WordPress Multisite." What happened, and how do I fix this please?