WordPress 3.5 seems to have broken my site

I downloaded the 3.5 version of WordPress straight from my dashboard. However straight away I could no longer access the admin panel but the front end looked fine.

I’ve followed the tips I’ve seen in these forums like disabling plugins and installing it manually but nothing seems to be working. But now if you go to my site you get this error:

“Fatal error: Call to a member function register_handler() on a non-object in /websites/123reg/LinuxPackage22/em/po/ri/emporiumofmanliness.co.uk/public_html/admin/wp-includes/media.php on line 944”

It also changes the url from https://emporiumofmanliness.co.uk/admin/wp-admin to https://emporiumofmanliness.co.uk/admin/wp-admin/install.php

That seems to tell me that it doesn’t think I’ve installed WordPress but I’ve used WordPress on the site for months before the update.