As it sounds like you haven't actually run any of the upgrade steps (you've just uploaded the files), restoring the files from the version you were running previously should allow the site to be accessible again.
Alternatively, you may wish to submit a ticket with login details (admin and FTP) and we can investigate, though it's worth mentioning that it's currently after 11 PM in the UK on a Sunday, so it's very likely we wouldn't be able to investigate until tomorrow.