have you tried:
-deactivating ALL plugins (yes, all) temporarily to narrow down the problem. If the problem goes away, re-activate them individually (one-by-one) to find the problematic plugin(s). If you can't get into your admin dashboard, try resetting the plugins folder by FTP or phpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems (because the hooks remain unless plugins completely removed or some plugins stick around in cached files. So by renaming the folder, you break them and force them inactive). If applicible, also remember to deactivate any plugins in the mu-plugins folder. The easiest way is to rename that folder to mu-plugins-old
- To rule out any theme-specific issue, try switching to the unedited default twenty twelve theme for a moment using the WP dashboard. If you don't have access to your admin area, you can switch to the default theme by renaming your current theme's folder in wp-content/themes via FTP or SFTP or whatever file management application your host provides.
- also review this troubleshooting 3.5 post: http://wordpress.org/support/topic/troubleshooting-wordpress-35-master-list?replies=4