Site is being upgraded - failed plugin install(2.5.4+)

Hi All

I recently attempted to install a plugin on our moodle 2.5.4+ (Build: 20140220) server - the installation appears to have timed out and I received a 500 error code. The plugin seemed to have actually installed and I assume the script took longer than the max_execution_time but, whenever I attempted to get to the admin notification page I would get the error: Site is being upgraded, please retry later.

However, since restarting the server ALL visits to the site now receive this message - is anyone able to point me in the right direction of where to start?

The server is running on Windows Server 2008, PHP 5.4.16, MySQL 5.6.