Internal server errors (error 500) are often caused by plugin or theme function conflicts, so if you have access to your admin panel, try deactivating all plugins. If you don’t have access to your admin panel, try manually resetting your plugins (no Dashboard access required). If that resolves the issue, reactivate each one individually until you find the cause.
If that does not resolve the issue, try switching to the Twenty Fourteen theme to rule-out a theme-specific issue. If you don’t have access to your admin panel, access your server via FTP or SFTP, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the default theme to activate and hopefully rule-out a theme-specific issue.
If that does not resolve the issue, it’s possible that a .htaccess rule could be the source of the problem. To check for this, access your server via FTP or SFTP and rename the .htaccess file. If you can’t find a .htaccess file, make sure that you have set your FTP or SFTP client to view invisible files.
If you weren’t able to resolve the issue by either resetting your plugins and theme or renaming your .htaccess file, we may be able to help, but we’ll need a more detailed error message. Internal server errors are usually described in more detail in the server error log. If you have access to your server error log, generate the error again, note the date and time, then immediately check your server error log for any errors that occurred during that time period. If you don’t have access to your server error log, ask your hosting provider to look for you.
Did you actually get a “failure” notice or error message? Or did it complete but now you cannot get in?
Those are two different scenarios and could be caused by different things – an update that completes fine but then leaves you unable to login *could* be caused by a Plugin or Theme conflict….if you have FTP access you can try renaming your Plugins folder first, and renaming the folder of the Theme you were using to force WP back to the default Theme.
If it simply failed and gave you a failure/error message, well sometimes the automatic updater fails….we all wish it wouldn’t, but it does.
Again, assuming you have FTP access to your site, I would recommend trying a manual update, it’s not difficult.
http://codex.ww.wp.xz.cn/Updating_WordPress#Manual_Update
Just be sure to backup your database if you can using phpMyAdmin first, and don’t overwrite your /wp-content/ folder, as that is where your Themes and Plugins are.
When I do a manual update, I download the latest version of WP, unzip it, and simply delete the /wp-content/ folder that it comes with, then upload all the rest. If you’re very brave and are careful, you can be sure you won’t leave detritus from the old version by first deleting all the files and folder in your WP folder EXCEPT the wp-content folder and your wp-config. file
ALWAYS backup everything first, but this should get you back in.
