It has happend twice now.
1. I get a report from one of my admins that the backend of the site is inaccessible.
2. http://siteaddress/wp-admin causes a 500 error.
3. I login at http://siteaddress/wp-login.php successfully but when I go to the backend (wp-admin) there is a 500 error.
I've got a plugin that allows me to remotely deactivate and activate plugins. I narrowed it down to the WPMUDev Dashboard plugin. After deactivating the plugin the site was accessible. I was then able to re-activate the plugin and it works just fine.
So...I'm not sure why but in the last week I've had this happen twice. Your WPMUDav Dashboard plugin is causing sites to be inaccessible until the plugin is deactivated and activated.