Help! Plug-In has disabled my site

I uploaded the Messaging plug-in, not realizing that it is a multi-site plug-in, and now my whole site is down. I can’t even access my WP dashboard.

Here is the site:

I contacted Fat Cow to help on the CPanel backend, and they told me that:

“The backup file of your WordPress website which we have in our server also shows the same contents. So, please contact the vendor of the application to investigate further on this issue. We support only for installation and upgradation of application.”

I would SO appreciate your help! I love WPMU and I look forward to integrating this plug-in after I have gone through and turned my site into a multi-site.