I couldn't install and activate the WPMU Dev Dashboard Plugin in one of my site.

Hello,
I have installed and activated the WPMU Dev Dashboard plugin in http://www.lifengolf.com. There was no problem up until now that I couldn't install that plugin in that site. I have many sites that have this plugin, but this is the only site that I am having trouble. It happened when I tried to update that WPMU Dev Dashboard plugin. Site had been down, and I had no choice but the delete that plugin.
Please guide me what went wrong with that site and find me the solution.
Thanks,
Austin