Going to /wp-admin offers to download the upgrade.php file which is weird.
Going to the upgrade.php page by typing it into the browser results in a blank page, but viewing the source indicates (or appears to indicate) that you have WP Super Cache enabled. Not sure whether this is causing the issue, but it could be a suspect - I'd always disable it before upgrading.
Can you ftp to your site and disable the WP Super Cache plugin by temporarily renaming the plugin folder then try to get in again?
FWDrew - if you're not going to try to help, why hijack the thread with unhelpful remarks?