New theme: Front end working / Dashboard down

Hi everyone,

I just uploaded the WordPress Bootstrap theme and activated it. I can access the public facing pages (e.g. http://www.pbnj.com.hk) but the dashboard (or any other backend page for that matter, especially the Themes section!) will no longer load (e.g. http://www.pbnj.com.hk/wp-admin/).

I get the following error message for any dashboard / backend page:

Server error
The website encountered an error while retrieving http://www.pbnj.com.hk/wp-admin/. It may be down for maintenance or configured incorrectly.
HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.

Any ideas how I can fix this? Not being able to access the backend is somewhat over my head. I checked on wordpress.org support but couldn’t find anything helpful.

Thanks and regards, Anthony.