Try the following steps:
– Deactivate all plugins and see if it helps solve the problem. If yes, reactivate plugins one-by-one to find the problematic plugin(s). If you can’t login to your dashboard, rename the plugins folder by FTP/ hosting control panel or PhpMyAdmin.
– Delete inactive plugins as they can also cause problems
– Switch to Twenty Thirteen theme to rule out any theme-specific issues. If you can’t log in to change themes, you can rename the theme folders via FTP so that only one is Twenty Thirteen that will be used by your site.
– Manually update from a fresh download of WordPress except wp-content – do not overwrite but delete wp-admin and wp-includes and replace them with fresh copies.
– Flush caching plugins, if any, and flush server and/or browser caches
– Run the upgrade manually via wp-admin/upgrade.php
Use FTP or your hosting control panel if you cannot access dashboard.
Backup your site first so that you can restore in case anything goes wrong.
