Nowadays it seems pretty routine to upgrade WordPress. Go into your WP-Admin control panel, click on upgrade WordPress Automatically and everything is downloaded, installed and upgraded without any issues…Most of the time.

I tried to upgrade a WordPress site the other day, though, and was met by the following error after the upgrade failed.

Fatal Error: Call to undefined function wp() in /home/mark/public_html/wp-blog-header.php on line 14

I could not get back into the WP-Admin control panel anymore and the site was completely down. I’d made a backup of the database before upgrading, but I wanted a simplier fix than starting over. Searching Google for answers proved to be more of a waste of time with lots of questions about the error but not really any solutions.

The key to my particular situation was found by FTPing into the domain and finding most of the wordpress files as zero byte files. In other words, the files were empty. However, the wp-config.php file was intact and still contained links to the database. This was extremely good news.

Sometimes with this particular error, all you have to do is add the following lines to the wp-config.php file to tell WordPress where the files are located. Substituting the term ‘’ with the actual domain name.

define('WP_SITEURL', '');
define('WP_HOME', '');

In my particular case, this was not necessary since the wp-config.php file was intact and unchanged.

Steps to Fix Failed Upgrade

Follow these steps to fix this type of situation.

1) Download the latest version of WordPress from to your hard drive

2) Extract it into a WordPress folder

3) Using an FTP program, log into your site and delete the following directories on your WordPress installation:

  • wp-admin
  • wp-includes

4) Now transfer the wp-admin and wp-includes folder from your hard drive to your site using FTP

5) Transfer all files in the wp-content folder on your hard drive to your site overwriting the files. Do not delete the wp-content folder from your site, just overwrite the files. WP-Content contains your themes, plugins, and more, so you do not want to delete these just fix the basic files in wp-content.

6) Lastly, transfer all the files in the root directory of WordPress on your hard drive to your site. Overwrite any files except wp-config.php.

7) Delete the .maintenance file from the root directory of your site if it exists. Many times a failed WP installation will leave this file and you won’t be able to see your site online until its deleted.

8 ) Now using your web browser visit the wp-admin page of your site. Ex.

9) Log in normally and WordPress may tell you that the database needs to be upgraded. Follow the instructions to do this.

10) Now check your site from a web browser and everything should be back to normal

If its not, please check your wp-config.php file for the correct information for your database including database name, username, password, and the correct path to the database.

Hopefully, after completing these steps your site is operational again!

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