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Have you tried…

• flush any caching plugins you might be running, as well as server and/or browser caches.
• deactivate all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s). Sometimes, an apparently inactive plugin can still cause problems.
• If you can’t get into your admin dashboard, try resetting the plugins folder by FTP or PhpMyAdmin (read this page if you need help).
• switch to the Twenty Eleven theme to rule out any theme-specific problems.
• If you can’t log in to change themes, you can remove the theme folders via FTP so the only one is twentyeleven. That will force your site to use it.
• manual upgrade. When all else fails, download a fresh copy of the latest.zip file of 3.5 (top right on this page) to your computer, and use that to copy up. You may need to delete the wp-admin and wp-includes folders on your server. Read the Manual Update directions first.
• check the Master List to see if you’re experiencing a known issue: http://wordpress.org/support/topic/troubleshooting-wordpress-35-master-list